There are several possible reasons. The most likely to me seems to be to do with mounting the bicycle, and when wheeling it on foot. The majority of people apparently mount their bike from the left side, and stand on the left when pushing it (something to do with right-handedness?). It therefore makes sense to put the transmission on the right, away from the rider's leg when standing beside the bike. Tandems of course usually have a chain on each side, and it's technically possible to set up a fixed gear with left-side-drive, though the pedal threads will be the wrong way round.