A little slow in replying. As a helicopter pilot I can say that sitting on the right gives easy access to use systems in the aircraft with the left hand while holding on with the right. As the machines gets more sophisticated you can let go, but the tradition remains.
Most aircraft commanders will assign seating position as to what suits them most, and the First Officer or copilot or whatever the pilot designation is, as long as qualified in either seat, can sit where ever.
In our operation (EMS) we switch out daily, so both get experience flying and making radio calls etc.
A couple helicopters have the captain or pilot in command sit left, such as the MD500, since you can eliminate the centre controls, and use that for a seat.