It's probably one of those questions that will never be answered, but good fun anyway. I'd always assumed that as the Airplane (spelled that way in deference to it's country of origin) was an American invention, and as Americans drive their automobiles sitting on the left, that it flowed on to aircraft.
In my own experience in a past life, when I would ask students to do a roll without specifying the direction, at least 90% did so to the left (and this was in tandem seating aircraft). Now, in a later life, when I ask students to do a steep turn in the simulator without specifying the direction, 90% will still go left, irrespective of whether sitting in the left or the right seat. Seems to work in Northern and Southern Hemispheres, so Coriolis can't be a factor.
Another mystery of aviation ?