Twice a lunar day the rock moves up and down, just as twice an Earth day the tides go up and down. It takes lots of energy to distort rock this way, and it comes out of the rotational kinetic energy of the Moon. So, the Moon slows down, and eventually comes to the point where one side always faces the Earth. The Earth is being slowed by tidal friction, also. I believe estimates are that it rotated in 20 hours or so when first formed, so it has lost 4 hours to the Moon's effect.
Smaller bodies have less rotational kinetic energy, so they become tidally locked faster.
Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.