Until the early 20th century, few restrictions on immigration to the united states existed primarily because

They were needed--Immigrants provided a source of labor for undesirable jobs and there was room for immigrants to settle.

Immigrants provided a labor source for industrialization or moved to the Midwest and became farmers who fed the country. Most early immigrants were from Western Europe and similar to Americans. After the 20th century, restrictions were put into place to keep undesirable groups out especially from Eastern Europe and East Asia.