"Early bikes had only a rear brake. In France, this usually was a rim brake. The early brakes were not very powerful, so you needed lots of hand power to stop the bike. Most people are right-handed, and it made sense to control the single brake with the right hand. In Italy and Britain, the single brake was a coaster brake, and there was no brake lever at all. When front brakes were added to bikes sometime in the 20th century, this required adding a brake lever to the handlebars. In France, the right side was taken, so they mounted the extra lever on the left. In “coaster-brake countries,” the handlebars were still empty, so the brake lever for the front brake went on the right side. When racers started using rim brakes on both wheels, the extra brake lever (for the rear wheel) went on the left side.

Many reasons have been put forward for the “right hand – front brake” approach. Most motorcycles use that configuration, since the right hand operates the throttle, the left hand the clutch, which in turn means that the right side of the handlebars is the only place to put a brake lever."

https://www.renehersecycles.com/whic...r-which-brake/