Have looked at previous thread and it is interesting that no one has mentioned the cargo/trooping door from early military helos.
I believe the tradition originates from the need for the Handling pilot to see the troops as they enter the disk (and to clear them in).
Again, only a guess but it makes sense, of course the door could have been put that side as the pilot was already there!!! (Chicken and egg etc)
Ask a 100 pilots etc!!!!
R1a