My guess is that the likelihood of falling on one side or the other is more luck than anything else. I don't fall often, but when I have taken a tumble, the circumstances were unique: Wiped out on a gravel path: right. Wiped out of a rain-slick road: left. Couldn't unclip in time: left. Front brake failure while cycling downhill: over the handlebars. Sideswiped by a pickup truck driver who turned sharply into the bike lane: managed not to fall. Sideswiped by a passing motorist who didn't bother to stop: kept my balance.