Because the satellites travel at high speeds around the world.
A GPS "client" on earth pinpoints its position by sending signals to these satelites and back and then compares the difference of time.
Because of the relativistic effects (observer on earth vs the traveling satelite), if you don't calibrate the internal clocks of the satellites to accomodate for these relativistic effects, the client positioning will be off by several miles. This because, relative to the observer on earth, time on the satelite flows at a different rate.
If the internal clocks of the satelites are calibrated to run at a slower rate as compared to those on earth, then it can pinpoint your position with an accuracy of 1 to 3 meters.
That's why.
That's basic relativity: relative to the observe, time slows down as speed goes up.