"...Homo-sapiens-only club."
"Present company excluded, of course."![]()
Best in-universe explanation from an outsider looking in. Of course:
-- I think there is an in-universe theoretical explanation for way so many species look humanoid coming from TOS, but I can't remember it. I just vaguely remember Spock mentioning it. Or maybe I'm dillusional and imagining it.
-- Some of the more "less humanoid" species may not be suited for space flight in Starfleet vessels, and so they serve the Federation in other ways. Economically, diplomatically, politically, and so on. They actually may be overrepresented in these areas and underrepresented in Starfleet.
Otherwise, out of universe:
-- You probably can't have a bunch of people (especially main characters) in make-up for hours a day. I'd imagine that would slow the shooting schedule.
-- There are only so many ways to make-up a human to look alien and still have that person be able to maneuver convincingly in our real world (in other words, the person needs eyes, a mouth, a nose to breath with, and will need to be bi-pedal). For example, let's see that guy in the bar scene at the beginning of ST09 actually get up, move around, and talk in any way that would be realistic.
In TMP they tried hard to introduce more "alien-looking" species into the background of the picture because they finally had a budget that allowed it. Several new species were introduced. (There were photos of some of them and the name of each species and brief description of them and their contributions to the Federation on the album jacket of the soundtrack.)
In TVH, you actually see quite a few less-than-humanoid looking species at the Federation Council meeting where Kirk was demoted back to captain and given a starship to command.