I seem to recall reading once that the first few pilots to fly 2-seat rotary aircraft were all experienced FW pilots used to sitting on the left - so they did. They then checked out the next generation, but hadn't really got the hang of flying helicopters yet so stayed in the left seat where they were happy.
So, the second (and rather larger) generation of helicopter pilots were taught to fly in the right hand seat, and the habit stuck.
This could of-course be complete urban myth - I've no idea, I wasn't there.
G