Well many prefer the Nintendo layout and are used to it, esp. for Nintendo games.

Interestingly, after a bit of searching, it looks like the Nintendo layout came first, but due to a patent or something, SEGA and later Xbox would swap the ABXY buttons. Not saying that either layout should be the “standard” though. Both should ideally coexist (and that includes the PS layout. You can’t really complain about the PS layout changing all the time, it’s basically the same since the original PS)

Also, what do you mean by A being the “default button”? Do you mean that A is for doing most of the interacting (e.g. dialogue, next, OK) or something like that? Because A does that in both Xbox layout (where you interact with the bottom button) and Nintendo layout (where you instead do that with the right button, at least on “standard” controllers. The GameCube controller kind of did its own thing)

Interestingly, it seems that some PS games interact with X and others interact with O. Apparently this is because in Japan, “O” means correct and “X” means wrong, while this significance of “O” doesn’t exist in the US and Europe, so “X” is accept because that’s where the thumb usually rests. Cool!