There seem to be four broad categories into which civilisations fall in the Trek universe. That is of course something of a simplification, but it will do for our purposes. The categories are :
|Pre-space flight civilisations. These would range from stone age societies through to those at a technology level roughly equal to Earth's early twentieth century, such as the "Romans" in TOS's "Bread and Circuses"..
|Early space flight civilisations. These are capable of simple space flight, but not warp travel. Examples would be the Valakians from Enterprise's "Dear Doctor", or the Malcorians from TNG's "First Contact". (the episode, not the movie).
|More mature space flight civilisations, with warp drive and interstellar colonies. Humanity would be crossing into this phase in "Star Trek : First Contact"; the Federation and other major Alpha Quadrant players would also be in this phase, as would species such as the Dominon and Borg.
|The super-aliens. These have acquired the ability to manipulate matter and space in ways not apparent to more lowly species. Examples would include the Organians, the Douwd, and ultimately the Q.
The simplest model of how a species would progress in Star Trek is to begin as a stone age culture, progress to using metal tools, machines, etc., then develop space flight, eventually invented warp drive and became the sort of multi-planet culture which the Federation or Klingons are, then eventually become a super-species.
The super-aliens in Trek don't seem to be technology users at all - rather, it appears that at some point most species undergo a radical single-generation mutation which gives them extraordinary powers to manipulate matter, space and even time. Yet this doesn't seem to happen to a species until it has passed through the other phases - that is, we don't ever see stone age primitives suddenly changing into super-aliens the way that we see spacefaring species like that in TNG's "Transfigurations" do. This change is usually - and wrongly - labeled as evolution, and apparently the writers are saying that intelligent species pass through the other phases as a natural progression toward the God-like status of the super-aliens.
Once a species has crossed this barrier it seems that further progress is limited to increasing the level of these abilities, with the Q apparently representing the ultimate development of life in the Trek universe.
Looking back at Human history, we first started tool-using some 2 million years ago. Phase 2 is vastly smaller - we moved into it in 1957 when Sputnik I was launched and on the Trek timeline we moved on into Phase 3 in 2063 with the launch of the Phoenix. Even if we say that the warp-5 capable NX class represents the first "true" Human warp ship, the whole of phase 2 still lasts less than 200 years.
How long Phase 3 lasts appears to be quite variable. As mentioned earlier, we saw a species undergoing the change in TNG's "Transfigurations". Those aliens didn't seem to be all that far ahead of the Federation, certainly no more than a century or so. On the other hand the Borg have been developing for "thousands of centuries", while the Voth species seen in Voyager have been in Phase 2 / 3 for approximately sixty five million of years.
We know that as of the TNG era Humans have remained Phase 3 two or three centuries, again depending on where you draw the 2/3 boundary. We don't seem all that close to becoming super-aliens - we have seen Humans from many centuries in the future who don't seem different from those of the present day, so the change is certainly not imminent. But looking at the rate of technological advance typical amongst Humans, it's hard to believe that we're still going to be in Phase 3 in, say, another ten thousand years.
Phase 4 has the most questionable span of all. It seems to be open-ended, i.e. once you have crossed the barrier to being a super-alien, you stay that way essentially forever. We know that the Q have been around for some billions of years at least, and there is no real reason why a super-alien should not continue to exist literally forever.
So while there is a lot of guesswork here, it seems that a species should spend a large chunk of time in Phase 1, virtually none in Phase 2, stay in Phase 3 for a while, and then stay in Phase 4 for the longest time of all.
If every species followed this path then the number of species in each phase would be directly proportional to the average time a species spends in each phase. This would mean that almost every species you would meet would be either a super-alien or a stone age primitive. Very few would be warp capable societies such as the Federation, whilst almost none at all would be Phase 2 civilisations capable of space flight but not warp.
This is a little simplistic, though, because not every species will make a progression through all phases and end up as a super-alien. Some will become extinct along the way, either wiping themselves out through warfare or ecological mismanagement, or being wiped out by others. Some will also be extinguished by natural disasters.
Once more, we can only guess about how common this type of thing is. Judging from Earth's history, natural disasters capable of wiping out large sections of life on a planet seem to be quite rare, one every few tens of millions of years or so, but Earth may not be typical. The planet Jupiter has swept up a lot of the asteroidal debris left over from the formation of our system, making impacts on Earth far less common than they would otherwise be. Systems with an inhabited planet without such a handy gas giant nearby might suffer impacts every million years or less. Civilisations on these worlds might never have time to make it out of Phase 1, if indeed they could evolve at all.
We have also heard of some species in Trek becoming extinct when their star went supernova. This must surely count as the ultimate natural disaster, but it would only come at the end of the star's life and this should, for a normal star, leave some billions of years for a civilisation to evolve and develop. Certainly our own sun's ultimate death is so far away (about four or five billion years) that we should have plenty of time for our 'evolution' to run its course in the meantime.
If it is hard to pin down how likely a species is to suffer a catastrophic natural disaster, acts of war are even more difficult to judge. We can say with reasonable certainty that no Phase 1 species should be able to wipe itself out in a war - the kind of world-wide weaponry and culture required just don't exist until Phase 2.
Once in Phase 2 a culture would by definition have the means to deliver weapons to any part of its planet, and should certainly have the technology to manufacture warheads with the yield to destroy cities en masse. At this point wars which could completely wipe out the civilisation become possible, and indeed even easy once biological weapons become widespread. This is, of course, the point at which Humanity stands right about now.
In Phase 3 the likely hood of destroying yourself would fall significantly. Although the weapons grow in power to a point where destroying a even a whole planetary surface is relatively easy, the spreading out of your population base to thousands of planets along with the large number of warp driven ships available ensure that even a massive war will always leave some survivors. One only needs to look at the El Aurians for a perfect example - even faced with the overwhelming might of the Borg, there were survivors who escaped and subsequently flourished. Guinan confidently predicted that in a worst case scenario Humanity could do the same. A similar argument applies to natural disasters - as of the 24th century rendering Humans extinct would take a disaster which could quickly destroy a large section of our galaxy, something which is unlikely in the extreme. Only deliberate action on the part of a Q-like entity could render a serious Phase 3 society extinct.
If we can only generalise as to how vulnerable a society is to extinction at each stage, we can say with certainty that this does happen in Star Trek, and that the effects would be to lower the number of Phase 2, 3 and 4 civilisations as compared to Phase 1. If anything this should mean that even fewer of the aliens we see in Trek should be in Phases 2 and 3, while Phase 4 aliens should be rarer but still very common.
Obviously, this is not the case. In fact most of the civilisations we meet in Trek are within a few decades one way or the other of the the Federation. The real reason for this is dramatic necessity. Writers can tell a far wider variety of stories if they are dealing with alien species similar to the heroes than they can dealing with either a stone age or a super-advanced alien. But we don't deal in reality here...
In fact, though, the apparent commonness of Phase 3 aliens can be explained fairly easily. It's not that the Phase 1s and 4s aren't out there in large numbers, but rather that Starfleet would rarely interact with either of them. In the TOS episode "Bread and Circuses" we get a description of the Prime Directive as it applies to a primitive civilisation :
|"Then the prime directive is in full force Captain."
|"No identification of self or mission. No interference with the social development of said planet."
|"No references to space or the fact that there are other worlds or more advanced civilizations..."
With these restrictions in place, there would be very little for Starfleet to do on such a world. We know that the Federation plants research teams on primitive planets to observe the locals, especially as they begin to move into space, but that seems to be about it. So you would expect a Starfleet ship to stop at such a planet only rarely.
At the other end of the scale, most Phase 4 civilisations seem to be pretty wary of interacting with us. The Douwd, the Organians and the Metrones all declined to reveal themselves to Humans until the Humans forced their hand in some way. The Traveler said that his people found Humanity too boring to visit until recently. In fact, of all the super-aliens we have seen only the Q seems to take any active interest in us, and even then it's only one of those. Given their abilities, it's certain that the Federation isn't going to be able to interact with any super-alien who doesn't want to talk, so it's little wonder that such contacts are rarely seen.
So we are left with a situation in which the Phase 1 and 4 aliens are the commonest in the Galaxy, but are the most rarely visited, while the Phase 2 and 3 civilisations are the rarest, but most commonly visited. And between the Phase 2 and Phase 3 aliens, the Phase 3 are going to be by far the more common. So we end up with a situation in which most species the Federation interacts with are going to be space faring warp drive civilisations - which is what we do indeed see on the show.
While we can make a good case to explain why most species that we see are in Phase 3, this only goes part way toward answering why those species are, in general, so exactly alike. We have already said that Phase 3 could cover tens of thousands of years; certainly it covers everything from at least the NX class Enterprise all the way up to Borg cubes and Voth city ships, or even up to the likes of the Dyson sphere builders from TNG's "Relics" who would be formidable indeed. Yet if you look at civilisations like the Federation, Romulans, Klingons, Ferengi, Breen or Cardassians, it is clear that their technology is restricted to a very narrow range indeed - certainly they are within a few decades of one another. And most other civilisations which we meet are within this same narrow band. How is this possible?
Again, the real reason is dramatic. Even a few decades worth of technological advantage would make one species all but unbeatably superior to another. If such species were around on a regular basis, it would be hard to stop the show from revolving around them rather than the Federation. When the Borg / Dominion / whoever invade the Alpha Quadrant, how could you not have the biggest, most advanced species around get involved in that?
However, as I said earlier I don't deal in reality. We need a Trek reason why there are few if any radically advanced Phase 3 civilisations around.
One possibility is that they have been wiped out. Assume that some terrible invader swept through the alpha quadrant; that the destruction was complete enough and long enough ago that few records survive about them. If we peg this as being say about a couple of thousand years ago, then we could speculate that the attackers left primitives such as Humanity pretty much alone, but eliminated all the major space faring species of the time. In the aftermath a new crop of civilisations grew up, all starting at roughly the same place. This would go a long way toward evening out large technological differences amongst the alpha quadrant powers.
It would still be unlikely that everybody would still be within a couple of decades of one another by the 24th century purely by random chance, but it's a little more believable than having the same thing happen after tens of thousands of years. Besides, trading between different species would tend to level out small technology differences anyway.
The obvious choice for this terrible invader might seem to be the Borg, but in fact they seem to have been victims of exactly this kind of attack themselves. In Best of Both Worlds, Guinan says that the Borg have been evolving their biological / technological mix for "thousands of centuries". Yet in the Voyager episode "Dragon's Teeth", Gaul says that the Borg were a very primitive species who nobody worried much about only a thousand years ago. This implies that the Borg underwent some sort of catastrophe somewhere in their history, and only began to recover about a thousand years ago. If this catastrophe was an attack - and it would almost have to be, since we have already established that any Phase 3 civilisation is all but immune to natural disasters - then the attacker would have to be extremely powerful to be able to smash the Borg down to almost nothing. Such an attacker should be able to wipe out Federation-style civilisations with ease.
Whilst we have to jump through a few hoops to explain the similarities in the technological levels of the major powers in Star Trek, I hope I've shown that it's not all that farfetched. The idea that the Federation is only likely to interact with other Phase 3 species is based on a mix of canon and simple common sense. Explaining why all Phase 3 powers are similar does need some speculation, but even that ties in nicely with some canon history.