When I was learning the basics of riding, I have this question.
Our instructor gave a very simple answer to it.
“Its easy to get on the bike from left side when its leaned over”
Most of the bikes has gears on left side, so when we sit on bike and start, we support the bike with right leg, then its easy for us to remove the stand and then engage the gear, while we are already supporting the bike with right leg.
New Usage: Modern days bikes also has a stand alert feature. If the bike is on stand and you engage the gear, then the bike will stop immediately (KTM bikes). This is possible because the stand was on left side.