Businessmen favored relaxed immigration laws because immigrants were usually willing to work for less money than American workers. Along with this, immigrant workers usually did not have a full understanding of the American economy and the wages that other individuals made for the same work.
Essentially, the businessmen could use immigrants as a cheap source of labor and were able to manipulate them. Immigrants often stayed in jobs even if they were treated poorly because they had no other options for jobs due to several different factors (language barrier, worried about supporting family, etc.).