I had this discussion a little while ago. Here was my submission to the debate
Is this because Cycles were invented when everyone rode on the left? This practice dates back to Medieval, even Roman times. Most people were right handed and when they passed people on the highways and byways they wanted their right side facing oncoming strangers in case there was trouble. Horses therefore carried their rider's sword on the right.
Once you have people all riding on the left side of the road, putting the gears on the right meant that the greasy paraphernalia was kept away from the puddles, overhanging shrubs etc. Also if work was needed, you simply lean the the bike against a tree or wall, rather than have to turn it around to face the way you weren't travelling. Plausible? N'est pas?
Hope useful.
Alfredo Marcantonio
Twickenham, England.