Qiyana versus Sun Goddess Karma

So, they actually already do have an in-game model that's far closer to a direct translation of Mayan/Aztec costumes than Qiyana.

It seems clear to me that the choice not to just make a copy of that skin was deliberate. Especially seeing as the feathered aesthetic is very closely tied to the belief system it accompanied. If the culture they're introducing doesn't have anything to do with bird-like gods, then there's no reason whatsoever to include feathers in the design. To do so would be rather insensitive to the real-world culture. Akin to including a bunch of crosses on a design for no reason. I'd be all for a Sun Goddess Qiyana skin, though, since the skins have their own separate universes.

A good example of a fantasy setting that uses Mayan/Aztec aesthetic but fits it in a way that makes sense is the dinosaur support in MTG. They've taken the real-world discovery that dinosaurs likely had feathers, exaggerated it a bit, and included a culture around it that incorporates those feathers in a similar fashion. League would have to introduce a culture that similarly has an actual reason to be using feathers. But barring copying the dinosaur approach, anything they do would just seem like an insensitive mockery of the referenced culture.

So they went with a safer base design. Of course, there's also the fact that they want to make base designs more generic so people will actually purchase skins. But you could say they made it too obvious this time.