There is no wind, no rain and no plate tectonics up there, so a four-billion-year history of what has been flying around this part of the solar system is still lying on the surface.

Earth has been hit at least as hard and has erased nearly all of it. The Moon has not: the crater record is what dates the Late Heavy Bombardment and what calibrates the crater clock used on Mars and Mercury. It is the only place where the early solar system's rubbish is still where it landed.