Why in two wheelers accelerator is on the right handle?

Because the clutch is on the left side.

Most people are right handed. Even car accelerator's are on the right.

People prefer it that way for standardisation.

It wasn't always the case. Early bikes had a throttle lever, or even a pedal.

Some early police issue Harley Davidsons had the throttle on the left, so the cop could shoot with his right hand.