The basic reason for most immigration to the United States during the period of industrialization was economic opportunity. Most immigrants who came to the US did so at least in part for economic reasons. The specifics differed depending on the region and the ethnicity of the immigrants, but the basic idea was generally the same.
Immigrants came to the East Coast and to the Midwest from Europe. Those who settled on the East Coast generally did so in order to work in factories. Those who moved on to the Midwest did so while working for the railroad or in order to farm. Chinese immigrants to the West Coast typically came in order to work building the railroads.
In the majority of cases, the real draw was economic opportunity. Immigrants have typically been drawn to the US in order to make a better living than they think they can attain at home.