I was told when I started flying helicopters (back around the Punic Wars) that the reason the HAC sat in the right was because all of the early helicopters loaded on the right side and the external hoists were on the right side. In addition the helicopters were marginally powered and could accomodate only one pilot. The pilot had to be in a position to see what was being loaded into his helo.
I have no idea what came first but I suspect the helo manufacturers said the door and hoist had to go on the right side and everything fell into place.
When the rear loading ramp and center hoist came in to being there was really no reason to change the arrangement.