Humans are a useful baseline species for any setting. The reason is simple – The audience is human, and they know what humans are like. There is no need to specify what colors their hair has, or how tall they are – we all know these things automatically. And we can more easily relate to humans.
On Earth, humans became the One Sentient Species because we’ve been the meanest, toughest, luckiest and perhaps smartest bastards around. We’ve outlived, exterminated or assimilated all competition. It stands to reason that either the same happened – or should have happened – in our fictional settings long before the onset of civilized society.
Does it have to be this way? Yeah. Kind of.
Let’s think of the alternatives.
One: We have company, and are slowly crowding them out. This is what happened in Earth’s past, and is really only the precursor to the above-described situation. Good choice for your average fantasy campaign – and this is actually the situation in Colonial Space.
Two: Humans are slowly being crowded out. In this case, it still makes sense to pick humans as the “baseline”, it just means we’re the Neanderthals of your setting. It’s a good choice and can lead to either a dystopian setting, or a “heroic fight for survival”.
Three: A balance of power. I don’t believe this is possible; sooner or later, one species will one-up the other. Cultural or technological sophistication don’t help much, either. We didn’t stop after we got rid of the other human species, we just turned on each other – if you don’t see it, ask the Native Americans, the Rwandans, the Tibetans, and so on. You get the point.
Humans in Colonial Space
Enderra was a multi-species planet. Humans, Endari, Mineons and Thurin made it off the homeworld alive; many others did not. The short explanation is that these four races had developed a series of historic alliances against common threats – and there has simply not been enough time for anybody to get the upper hand; as illustrated by the Endari’s possible off-world origin.
Let’s put this into in-Universe terms:
Humans dominate the major interstellar nations of Colonial Space. This is not due to their greater intelligence or physical strength; indeed, comparative xenobiology shows that humans are mediocre in both aspects. Instead, humans have a much higher reproduction rate than the other Enderran peoples. More children who reach maturity quicker has two effects: One, humans can adapt more easily to changing conditions (both in terms of biological evolution as well as cultural change) and two, it means that human society can more easily absorb deaths caused by hostile environments and wars.
This dominance is not a new development of the space age, but was already in effect back on the original homeworld, Enderra, where most powerful nations were ruled by humans. It was humans who led the age of discovery in their sailing ships and colonized new continents. Humans spearheaded space exploration. Humans led the Continuity Project, and were firmly in control of Fleet Command and the colonial administration. Both Empire and Republic are ruled by humans.
Short and to the point, and we’ve got a rationale that’ll work.