Test pilot
I heard it was because one of the first Test Pilots for Skiorsky (Lee Morris) in 1942-43 learned how a helicopter flew from the left. Then when he had to teach someone lese he didn't want to have to relearn how to fly from the right. So the students all learnt from the right and never knew any different. Later, it was safer and easier for a right hand seat instrucotr to train new people from the left due to higher experience levels.
All this is from the legend that is Ray Prouty.