I'm not sure what the optimal / recommended specs for are for keeping alligator snapping turtles, but I've seen them kept in much smaller tanks than 4x4 m.
True, I've seen them too. Again a debatable animal, I know. But as I said earlier I guide myself by the principle of flexibility: as long as there is a substantial amount of people that think it could potentially be a habitat animal (if it's reasonable, of course), I'm not the one who will say otherwise. And based on the previous meta-wishlist thread, ~20 people thought so.
It's a similar debate with iguanas tbh. Quite some people think it could also be a habitat animal, and it is reasonable because, like alligator turtles, in some (or many) zoos they're indeed kept in bigger ceiling-free exhibits or terrariums.
As a fun fact, modders even took ingame exhibit iguanas and made then habitat animals
This isn't an objection, but I guess the fact that the two tree kangaroos are different species and the leopards aren't might count for something. I mean, personally I'd take either (though I don't see them as essential, hence their absence from my list).
Edit: In fact I'm more particular about what leopard I want than I am about the tree kangaroo.
That is indeed the reason.
I acknowledge the fact that people (including me) might be more particular about leopards than about tree kangaroos. I even separated the leopards in the first meta-wishlist thread, but then some other people questioned whether they should be all combined.
As I wrote in the OP, animal species is the preferred category because this way each animal is "treated" equally and it's scientifically accurate.