Americans started to move to the West for multiple reasons. The land to the West had just been obtained by the United States, so it was pretty untouched and uncultivated. A lot of people saw this opportunity to start fresh and wanted to move.
The government started giving out very cheap, or even free land to the West, thanks to the Homestead Act. This interested a lot of people, many leaving their jobs in the North to become simple farmers in the West. The land and soil was great for farming, as it hadn't been overused and drained of its nutrients.
During the California Gold Rush, many people started to move West in hopes of finding gold and becoming rich. A lot of people ended up doing this, earning many the name "49ers" because they came during 1949. Not only Americans took this offer, but many immigrants did as well.