Just because. It's tradition. It's also easier. Why a fixed-wing pilot sits on the left has always stumped me. It requires flying left-handed much of the time, because the throttle(s), mixture, radios, and everything else are on the right. If you're in the right seat, everything is on the left, so you can use your left hand to do other stuff while you maintain control with your right.
There are a couple or eleven threads on this subject in the archives. A quick search should give you a few days' reading.