In the uk, we have the rear brake on the left so that when turning right and doing a hand signal with the right arm,, you can apply a gentle brake to steady yourself. If the left hand operated the front brake, it could be too fierce and easy to go over the bars. (I have proven this!!).
Interesting though, if you are a fixie and only have a front brake, it is customary to have a left hand front brake. Google some images of fixie riders from the old days. So fixie riders can apply back pedaling for a gentle braking, can still indicate with the right arm and if needed can still use the front brake.