Because it take a lot of energy for a solid body like the moon to deform as it should because of the tidal forces from its nearest neighbor. That energy gets taken from its spin, a tiny bit for each time it revolves, until it finally end up with a spin time equal to its obital time going around its nearest (and most influental) tidal force generator. Here on earth tidal bulges are a lot easier to make and cost very little energy (ocean water flows easily). So even though the rotation of the earth are slowing down a tiny bit every day it will take billions of years before we get tidal-locked to the moon.
Patchouli 