What caused the large drop from 1930-1950?

After their respective countries had faced many changes and economic instability, about 15 million Europeans immigrated to the United States between 1901 and 1910, a number that doesn't even compare with previous decades. The effect was greater because many people were escaping from prosecution, seeking better land or for better job opportunities and wages, which weren't always obtained.

The large drop from 1930-1950 was caused by the increasing policies, such as the Immigration Act of 1924, that were limiting the immigrants entering the US (especially Southern and Eastern Europe) and banning Asian immigrants, due to factors such as the belief of maintaining the American population homogenous, the stock market crashed which started the Great Depression, as well as the WWII, which was fast approaching.

As the United States' economy was increasing and stabilizing, it welcomed about 20 million immigrants.