Because, much like motorcycles, someone built their bicycles that way. Same reason why some motorcycles had foot shift, others had hand shift, some on the right, some on the left, some had first gear up - 4 down, some had neutral at the bottom - 4 or 5 up, some had rotary shifting going n 1 2 3 4 n 1 2 3 4, some actually had front brakes on the left when there were foot clutches. Until something becomes regulated, there are no explanations. Why do some countries run on the left side of the road while most go on the left side? Some people started it out that way. There doesn't have to be any reason. The only reason why most bikes are set up the way they are now is because of federal regulations in a number of countries. I am not sure of it, maybe someone works in a bike shop can tell us if a new bicycle has to have the brakes set up with the front on the left, rear on the right required by gov't regulations.