Need help on this ?...please!?

Your question seems to indicate that you believe that these pieces of material have a purposeful destination, the earth. They do not. It is equivalent to asking how a particular raindrop hits your head as a final destination. Gravity is pulling them towards the earth, but there is no targeting involved. The alternate question is why most space objects do not hit the earth or why most rain drops do not hit the top of your head. The misses are just as devoid of purpose as are the hits.