Safety bicycles were introduced into a world of horse riders, and most horses are, in fact, traditionally mounted from the left in the industrialized countries that pioneered bicycle manufacture & riding. People naturally begin to mount bicycles the same way they'd always mounted horses. Manufacturers put the drive train on the 'off' side, away from the riders' clothing. To facilitate left-side mounting, kickstands and the mounting step on ordinary bicycles (highwheelers) are always on the left, too.

Why are/were horses mounted from the left? Because most people are naturally right handed, and in the 19th century lefties were forced/trained to be right handed, too, so back in the day it was nearly a 100% right handed world. So, if you're right handed, where do you wear your sword, Mr. Man-of-action-and-adventure? (FYI - Cavalry sabers were issued into WWI.) That's correct, you wear your sword on your left hip. Now think about it: If you mount from the right, swinging your left leg over the horse's haunches, the sword...can and will wind up between your legs!

Now you know.