Why did the U. S. refuse to allow refugees in the US in 1939?

In late 1938, 125,000 applicants lined up outside US consulates hoping to obtain 27,000 visas under the existing immigration quota. By June 1939 ... In a highly publicized event in May-June 1939, the United States refused to admit over 900 Jewish refugees who had sailed from Hamburg, Germany, on the St. Louis.