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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.
It thought of that but the old Sega controllers were ABC XYZ.
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.