Why does the Moon have so many craters?

Earth gets hit by space rocks too, but our thick atmosphere burns up most small objects and weather, erosion and tectonic activity gradually erase impact scars. The Moon has almost no atmosphere, no water and no plate tectonics, so craters from 4 billion years ago look nearly the same as the day they formed. The heaviest bombardment happened during the Late Heavy Bombardment, roughly 3.8 to 4.1 billion years ago, when the inner Solar System was still full of leftover debris. The largest lunar crater, the South Pole–Aitken basin, is about 2,500 kilometres wide and 8 kilometres deep.