Probably because they're not required to do much in the way of hard braking... that's what the fronts are for. The rear is mostly for adding stability in low speed manoeuvring. It does come to play in a braking situation that's not entirely in a straight line (again, added stability) however, just remember when you're braking really hard, the weight transfer takes so much grip away from the rear tire that the rear brake ends up just being along for the ride anyway.