There are more craters seen on the moon than on the Earth because...?

Well, there's no erosion on the Moon, so that's probably what your teacher is looking for. The last one isn't true all the time, but the Moon DOES do an effective job of shielding us from a lot of impacts. Also, the Earth is molten inside (unlike the Moon) and will 'fill in' large craters with lava (or sometimes water, such as the Yucatan).