The shifter on a motorcycle is actually operated by the left foot and the clutch looks like a brake lever and is on the left hand. So shifting is done on the left side. The rear brake is foot operated by the right foot, so braking is done on the right side.

The throttle (accelerator) is a twist grip on the right hand, so when slowing down you often have to modulate the front brake with the right hand while simultaneously "blipping" the throttle to ease downshifting. One or two fingers on the brake lever the rest curled around the grip.

Your preference makes sense in the context of braking and shifting simultaneously with different hands, but I guess motorcyclist are used to downshifting and braking at the same time time with the right hand so that feels more natural to them.