I'm growing more and more convinced that they're intentionally avoiding adding some of the most requested animals either because they want to save stuff for a potential sequel. Like, I imagine they could have very easily added the Howler Monkey instead of the Saki, I can't imagine it would be that difficult, but they didn't. They have technically added another South American primate, which people have been clamoring for, but not the most requested one, which is odd. Same goes for South American species in general, they did hit a lot of requested ones like the Ocelot and the Rhea, but not the ones highest on most lists like the Mara or the Coati.
My guess is that they're trying to meet peoples' wishes (South American animals, birds, primates) enough to get sales, but not enough to where they won't have anything enticing to sell the next game.
(Or they're just doing the bare minimum due to having less staff but still having the ability to make profit off the game, and a potential sequel isn't even a factor. Which is probably the case.)