From what I remember, the moon has little to no atmosphere, this means that if an asteroid came hurling towards it, there would be nothing between it and the asteroid, so it would just come crashing down to the moon. Whereas the Earth has a much thicker atmosphere than the moon and if a asteroid came hurling towards the Earth, it would most likely burn up or break apart in the atmosphere before it reached the earths surface.
I hope this helped! :-)
