If the chain were on the left side, the freewheel (or single sprocket in the days before freewheels existed) would have to be left-hand threaded. Yes I know the left pedal already is, and the right fixed cup often is, but reducing the number of non-standard threaded parts when there was no compelling engineering reason to use them was probably sensible. Note that with freehubs you wouldn't need left-hand threads to put the drive train on the left side. Or was that a stupid answer owing to forgetting something?