Hangover from aeroplanes, throttle controls in a side by side configuration are in the middle of the aircraft hence pilot sits on the left,
The first throttles were attached to the port side of the cockpit. When an assistant was put in beside the pilot the pilot sat on the left hand side. and this continued. Mosquito, etc. Centre throttles when there were two pilots was easier but training aircraft had left hand throttles for both.
When Sikorsky built the first wide cockpits he put the pilot on the right hand side because all the collective paraphernalia was in the middle of the aircraft. Bell, using a lighter system put theirs on the left. Fashion made Bell change to the right but Russian helicopters still have the pilot on the left.