Due to the GamePad's large size, Nintendo used X as the jump button because it's closer to the ZR button.

I know right, but it feels alright cus the gamepad is huge, and its closer to ZR.

The X button is the closest button to the right analog stick, that way players can have an easier time jumping, shooting and stick aiming.

on the switch, the jump button in splatoon 2 & 3 are mapped to B because the B button is closest to right analog stick.

Also, the Joy Cons are tiny. And the Pro controller is much smaller than the GamePad.