Just found this:
Do you know why this is? A brief history of time…
Cycling hand turning signals originated from the same signals that cars used before they had indicator lights. For example, in America when a car wants to turn left, the driver (who sits on the left side of the car) put his left arm straight outside the window. If the driver wanted to signal right, he had to make a “L” with his left arm out the window (because he wouldn’t be able to reach out to the right side of the window with a straight arm to indicate a right turn). Similarly in places like Australia and the UK where you drive on the left side of the road, the driver (sitting on the right of the car) would have extend his right arm straight out the window to turn right, and same arm in a “L” shape to signal to turn left.
These turning signals were adopted by cyclists and the national standards organizations in each country got involved. There was concern that the cyclist should be able to make hand turning signals and still be able to reach the primary brake. The logic is accompanied by the premise that the rear brake is the primary brake. These standards organizations misunderstood braking and thought that using the front brake was hazardous and would cause the cyclist to abruptly topple over the front when hitting the front brake (endo!). In reality, very few accidents result from braking from the front.