As mentioned..
You have a ship with a Human Centrist life support and layout. Some species work well in that environment. Some need extra life support systems like a breather, or a chair. while some species can't operate at all in that environment.
I'm pretty sure there have been a number of aquatic species discovered. They can have areas, like Ceatation Ops, or they can have a whole ship with other aquatic or amphibians species.
Certain ships, like say the Enterprise is a plum assignment, and other speices may endure some extra burden to serve on her. but otherwise, wouldn't bother.
Another explanation would be, Humans are young, and want to explore, some of the older species may have already done there exploring, or are just not interested. You get the ocasional person that joins, but in all, they may join there own ship service. I doubt there is a "Quota" of people they must recruit. exept in war like the Draft.