Because most of the objects in our solar system are on the same plane of rotation around the sun. Planets are all very closely aligned, most of the objects that could impact Earth are on a similar trajectory. If an object hits near the equator, the faster and deeper it dives into the atmosphere, generating heat and ablating itself before it hits the ground. Objects etering near the poles have a longer time to slow down in less dense atmosphere. Less likely to burn up before reaching the ground.

That is a huge guess and not statistically sound.