As an ex-SAR pilot flying basic Whirlwind HAR 10 (S-55) a thousand years ago, we were taught that the elementary design of early helicopters led to a hover which was left wheel low because of tail rotor roll ( which conteracts tail-rotor drift)- all this because of the direction of rotation of the main rotor on early helicopters.
To counteract this unbalance, the winch (and the single pilot), were seated on the right as opposed to the left on fixed-wing aircraft to restore a wings -level situation.
Subsequently, the world's operators have continued to feed of this situation and now common practice decides that the captain sits on the right.
Simple really.