An odd choice for the MMO is that only humans and Irathients (the species that Irisa, Nolan's daughter, is) are playable. One would have at least assumed the Castihans would also be playable given the attention the pilot gives to Datak Tarr and his family; while Indogene are likewise humanoid enough to work (and the Sensoth and Liberata fit standard MMO body types).
More player races will be added in the DLC. (Not quite full-fledged expansions, as you posit, though I guess if you buy all of it, the price will be the same.)
So far Indogene, Sensoth, and Liberata exist as NPC character races. Castihans could exist too - I may have mistaken them for human.
It's also possible the game decided to stay away from them for now because
of all the attention being paid in the show proper.