A better question would be why Microsoft went with a nonstandard layout when they designed the Xbox controller. Nintendo had been using the A-to-the-right layout since 1990.
Sega consoles used the ABC/XYZ left to right format. If you assume X and Y are axis, then X on the left (horizontal) and Y on the right (vertical) makes more sense than Nintendo's Y on the horizontal and X on the vertical.
I mean it kind of, instinctually makes some level of sense to me. With nintendo's stuff, the A button is on the right, and the B button is on the left, so you're reading it right to left, instead of left to right. Hence, the accompanying swap in X and Y.
Playstation still basically conforms to a on the right, since O is generally used as a "confirm" button, and X is generally used as a back out button. So, they fill the same role as the conventional layout, they just abstract it in a kind of more fun way.
Playstation games over the years have used X for confirm in many (western) regions. I’m not sure the origin of this but it was always that way growing up.
Circle was used in Japan. Localisations like Final Fantasy using circle were the exceptions to the rule (I guess it was too hard to change it?)
The real question is why they deviated from the GameCube controller layout. Throws me off all the time when learning a new Switch game. "Y is on top" is something deeply ingrained in me from those days.
Real talk, I have never met someone who didn't own a version of the DS. It was such a huge thing at school and being out and about, especially collecting street passes with friends/kids you'd just met.
Sorry if I came across as a dick. Just genuinely didn't even consider someone might not have grown up with a Nintendo handheld of some sort lol.
I always assumed it's because they are from Japan and there they have a lot of things right to left. For them it may seem natural to start at the right and go left.
The real answer is that nobody knows because nobody wrote it down.
The most likely reason is because the old game and watch was a single button where the A was on the original NES (famicom) and that was the primary button, then the secondary button, or B, was placed slightly inward where it was assumed it would be used less.
It's a long enough video and just goes through the history of different layouts for different controllers and tries to reason why they are what they are.
I think the real answer was actually Japanese reading right to left, and applying that to alphabet buttons. What I find more interesting is the insistence that A must be the Accept button and B the back button; Nintendo games and OG Japanese games in general tend to use that layout, including PlayStation X and O (which to be fair makes even more sense for no/yes). US games afterwards flipped out, even for PlayStation games.
Really, Microsoft changing that up is genuinely evil to anyone already gaming, although I believe Sega also was left to right, but their three/six button layout doesn't count. Not sure how they handled accept/back though.
Sega had the dreamcast with an "A on the bottom", basic xbox style layout about 3 years before the xbox came out, as an extension of their genesis six button layout. With how friendly sega has been with microsoft historically, and especially the similarities between the classic "duke" controller and the dreamcast controller, the increasing focus on online play, I think maybe there's a through-line from the classic sega button layout and the modern xbox button layout.