OK, I'll take the bait. When the first primitive bicycle, the Pedestrian Hobby-horse, was first seen in 1816 in Paris, everyone reasoned that since you mounted a horse from the left side, that was the appropriate way to mount a Hobby-horse. When the chain driven was introduced in England ca. 1879, mounting a bicycle on the left side was further reinforced by the greasy chain that everyone wanted to avoid. Since horse racing goes counter-clock wise, I suppose bicycle racing on the track followed that tradition.
New York, NY