The size difference would be the main reason. On earth, the dense material is much deeper, so you'd need a lot more force to bring the it to the surface. The in

The size difference would be the main reason. On earth, the dense material is much deeper, so you'd need a lot more force to bring the it to the surface. The increased size and soft earth probably absorb more of the shockwaves as well. Having no atmosphere means that even smaller impacts will reach the surface as well. There are probably some impacts that have caused this on earth, but the difference in gravity is probably so small, that it doesn't effect orbits. (We could probably measure it though).