It's a mix of big, scary, unique, lively, iconic. Most large animals spend most of their time lounging around, so it's hard to find an animal that ticks all boxes. Maybe Polar Bear? Lions aren't (usually) lively, Meerkats are lively but aren't big or scary, Giraffes probably aren't considered overly lively or scary. Then you have some like Pallas's cat which kind of don't tick any of those boxes and therefore wouldn't be a huge draw to a zoo. I'm guessing just about everyone on this forum would be massive animal nerds, so for us it would still be super interesting to see, but most of the general public would think it's pretty much just a regular house cat.
I'd assume frontier would (consciously or subconsciously) judge potential new inclusions against these, or similar, criteria, as well as some others like ease of programming, popularity amongst players, and perception of "clone" status.