I remember posting this same question some months ago. Someone at that time said that when helicopters first came along, the first people to fly them were f/w pilots, so they sat in the left seat and their students sat on the right. So the first large group of heli pilots got used to sitting on the right, and carried on. People also mentioned not having to climb over the collective when you got in, and a whole host of other reasons, including the fact that some helis are flown from the left or the middle. Seems like the real reason, if there is one, is lost in the history of helicopter flying.