What economic and social forces drew people to the west

Well land for one in the west was a lot cheaper and available for those who migrated west. there were also farming opportunities. the east was also very crowded and became more developed  and the social forces are,  
a lot of the people in the east had family that would write from the west convincing them to move and take the advantage that the west had to offer.

hope this helps   

Native americans is the answer and have you ever heard of google