Orbiting around a planet is a very large space mall. It exists as a refuge away from planetary law where aliens from across the local group could gather together and sell to others of different alien races without worrying about the laws of their native planet(Kinda like a black market).
This opportunity isn't just reserved for the buying and selling of objects, and that is where the zoo comes in. Aw planetary law always keeps the interesting creatures out of the reach of the public, because of dumb reasons such as, they're endangered, can't handle captivity, or need the tourism to fuel their planets economy. So here, if you can get them in, the creatures can stay no matter how illegal.
And my question is basically what's a good reason for the owner of one of these zoos get a multi-species crew instead of just them all or most of his species, and instead have a diverse cast of alien races working for his zoo?