Traditionallly, side by side cockpits are flown from the left seat. Tandem cockpts in military aircraft are flown mostly from front seat (both of these statements refer to fixed wing only). Small tandem civil aircraft, because of CG factors may have pilot fly solo from rear (Cub) but others flown from front seat (Champ).
By and large if you think about how ground vehicles are driven- and which side of road to use- traditions developed over millenia, and are not the same from region to region, but in any one region, people find they need a de facto standard and one evolves.
Because ships visited many countries and regions, sailors first needed to establish international standards, and they did pretty well developing laws of seas.
ICAO has done the same thing for rules of air traffic, and is trying to establish airport conventions, but this is not as far along as air traffic rules.
BTW, don't forget 727, which had centerline boarding, :-)