There are many factors on why people decided to move West. People decided to move West because of:
Cheap land - Since the land was very cheap at the time, someone could get a lot of land for a cheap price and profit from it.
Rail Road - While the rail road was expanding during their time, it made it a lot easier to get supplies
Farms - There was a lot of land for farms to be built and make business, and when there are farms, there is people that have the opportunity to get work, people were totally okay with working in farms
Adventure - There is a lot of land in the West, and the people wanted to be free and explore land that they're not familiar with and find something knew.
There are many other facts on why people decided to move to the West, but those were just a few.