I recall reading somewhere that the standardization of designing the aircraft in order to board the pax on the left side and everything else on the right started because it was easy to park the aircraft on the ramp, on the old kind of airports, with the left side to the ramp, because you had a better look of the fence so all the aircraft manufacturers started to built them like that.
Regarding why the helicopters, or at least some of them, I heard once an interview made to Igor Sikorsky he said that as he was the first helicopter pilot, he sat on the LHS and then he trained somebody that seated on the RHS, then this person trained several instructor for the armed forces and all of the were seated on the LHS, and those instructors that were used to flight and instruct on the LHS, had all of theirs students seated on the RHS, so they get accustomed to be seated there.
Got it?
Like always, everything about helicopters is confusing starting with the way they flight!
