Hate to argue with Alex but the gear lever on the right had nothing to do with riding on the left side of the road.
Its very simple. Very early motorcycles had a direct belt drive from the engine to the rear wheel on their left side. As transmissions, chain drive, clutches and gearboxes etc, were developed the drive to the rear wheel stayed on the left and the means of changing gear was installed out of the way on the right hand side. This was practical as hand change was employed in those days and as most people are right handed it suited nicely to have the gear change where it was natural for most people to use it.
When finally the current style of positive stop foot change was invented in the late 1920's it just replaced the right hand side hand gear change that existed before it. It was an option you bought that bolted onto the right hand side of the box where the hand change entered the box. This caused new problems because at that time most bikes had the rear brake on the right hand side as they are now so the lucky rider with the new fangled fancy foot change had both to brake and to change gear with the right foot and do both at the same time. Obviously this didn't work that well so the makers took the cheap way out and just simply transferred the rear brake across to the left hand side.
This arrangement lasted from the 1930's to the late 1960's when it became more and more obvious that British motorcycle manufacturers had to get in line with the rest of the world and have the foot brake back where it was operated by the right foot like it is in every other vehicle on the road. Cars, trucks.buses, you name it, whatever you drive, the right foot works the brakes. To do that the foot gear change had to move across to the left.
So the old British style of controls was just one of the many steps in the development of the modern motorcycle and thats all there is to it. It met the needs of its day but then things moved on.
Cheers,
