The short answer is that the moon takes the same time to spin on its axis, as to revolve once around the earth. This apparently unlikely situation is stabilised by the action of tides, both on the earth, and in the lunar crust. These tides are, in effect, simply, gravitationally-caused bulges. It is likely that the moon once rotated much faster, but that the tidal effect acted as a brake to slow it until it reached its present rate of rotation, when there is no longer any tidal friction.