And buffalo, diamond dogs, donkeys, cows, sheep, horses from Saddle Arabia, etc.

I think it's also just because we've not seen anything about their homelands. We don't know anything about Zecora's people, Saddle Arabia or even where minotaurs come from. Hell, they specifically reference the movie where they meet all kinds of different species, like anthro-parrots and cat-people.

That, and they seem to want a heavy focus on non-equine based animals.

Though, I still want to see zebras make an appearance somewhere before this series comes to an end.