Because most people feel that the rear break is more for speed modulation and the front is where you should derive most of your stopping power. Part of that is due to no matter how you stop your weight will move forward when you hit the brakes. If you mainly use your rear, that forward shift in center of gravity can lead to partial unweighting of your rear wheel which could lead to a skid out and not so pleasant consequences. That same forward shift in center of gravity will put more weight on the front tire and lead to better stopping (assuming you do it right and don't go over your bars as you fear).