One more consideration. Back in the day (I'll use a 727 here) a lot of airports had ramp pax loading (i.e. you walked across the ramp, and up a stairtruck). On the right side of the aircraft were the refuel point/control panel, the cargo pit doors, and also as stated before, galley door, and the APU exhaust. So it was safer and more operationally efficient to load on the left while ground service operations were going on, on the right.. I guess it became more standard industry-wide when the various airport authorities put in jetways.