That's physics for you. Momentum (or Newton's 2nd law) causes weight to shift to the front as you decelerate. This increases maximum-grip possible from the front-tyre while decreasing grip from the rear. Depending upon the height of your centre-of-gravity (COG), the wheelbase and deceleration-G, at some point as braking-force increases, ALL of the weight will be on the front wheel and zero on the back:
This is when you have maximum deceleration force and shortest braking-distances possible. As a test, go out and practice and measure maximum-braking distances between front-only versus rear-only brakes. Measure the distances between the two and see how effective the rear-brakes are for maximum-braking force. Also practice braking with your hands in the drops, arms aimed straight-forward and your belly on the seat.