It's because of Tidal Locking.
Both bodies (moon and earth) gravitationally attract each other. This causes a tidal 'bulge' on the side of the body facing the other. Since the Earth is more massive than the Moon, the effect is greater on the Moon. Essentially the moon looses angular momentum due to the bulge, and slows down until the rate of roatation matches the rate of movement of the bulge around the moon (which is the same rate the Moon orbits the Earth).
The same thing but taking more time is occurring with the Earth, so eventually it will be tidally locked to the moon too, and the same side of the Earth will always face the same side of the moon.
Hope that makes sense!
EDIT: haha beaten to it
Jay