Who's to decide what's a "staple" animal and what's not? In comparison, ZT2 had no orangutans, indian elephants, tapir, giant tortoise, or asian rhino species. All of these came in expansion packs. Frontier kept some popular species for DLCs, but also put a lot of popular animals in the base game, which could've been kept for a DLC.
If they'd put only the most popular animals in the base game, you'd lose both the biodiversity and conservation of more niche species, and you could never make a satisfying dlc, because all the headliners would've already been present in the game from the get-go.
Had that been the case we'd be having a "birds of southern Quibec" situation, where the DLC would be niche and people would complain over the lack of popular animals, which would also kill the longevity of the game support. The game needs profitable DLCs for it to be supported.