Question

The foreign-born population in the united states reached an all-time low of 5 percent in 1970 because

Answer 1

It's because of the creation of national origin quotation system.
This regulation is created by the national government to reduce the amount of unemployment in the country because the locals simply wouldn't be able to compete with immigrants in the term of salary. By restricting immigrants influx, the job market is forced to target the natural-born citizens