Interesting. This was in 1952? Cold War stuff . . . I bet. The bookends of the period of low immigration are the Immigration Act of 1924 and the Immigration and Nationality Act  of 1965.  

The latter more or less ended the former, doing away with preference quotas that had favored western European immigrants--mainly white folks, of course-- from generally prosperous countries for 40 years. According to the source below, under the 1924 law, 70% of immigration permits went to folks from Ireland, Germany and the UK. The 1965 law also raised limits on the overall number of immigrants, I think. 

Good timeline of 20th-century immigration law here, from Pew

http://www.pewhispanic.org/2015/09/28/chapter-1-the-nations-immigration-laws-1920-to-today/