If I'm thinking of the right time period, they moved west because the east was overpopulated and they needed more land. They ended up purchasing a very large portion of land that doubled the size of the nation and caused some to believe they had too much land to protect from enemies (I think it was Britain).
Since it's a large time period, I'd just say both expansion and to find better jobs. There was another movement where people moved either east or west to find new jobs.
