Why did the number of immigrants into the U.S. increase after 1930?
I believe that you must have made a mistake in copying this question. Immigration to the United States did not increase after 1930. In fact, it decreased significantly. The reason it decreased significantly was the Great Depression.
You can see that the number of immigrants entering the US declined dramatically in the 1930s. The link below tells us that about 4.1 million immigrants came to the US between 1921 and 1930. By contrast, only 532,000 came between 1931 and 1940. This means that immigration in the 1920s was about eight times higher than in the 1930s.
One reason people migrate to America is economic opportunity. During the 1930s, the Great Depression was happening. This meant that the US economy was terribly weak and there was little opportunity to be had. This would deter people from coming. Because our economy was so weak, very few people, relatively speaking, immigrated to the US after 1930.