Comment by keyringlight
5 hours ago
One big reason would be that the 360 controller was when they first made it standard USB to connect, and introduced Xinput with the standard set of inputs for games to target. I expect most gamers wouldn't find it pleasant if they had to assign buttons and axis before the joypad would be active/useful, then hitting play and trying to remember what JOY_5 mapped to as used to be needed with directinput.
The number of sticks and inputs hasn't changed much since the XBox and PS1 days either though, it's not just that the 360 and XInput became a default. Outside of Nintendo's experimental time in the Wii and GameCube era it's been the default for several decades and even Nintendo has basically given up and come to the same format since about the Wii U days.