The Immigration Act of 1924 was passed because Americans had become worried about the levels of immigration and the types of people who were coming in as immigrants. The Act was passed to reduce immigration and to try to stop so many of the "wrong" kinds of people from coming.
The Immigration Act of 1924 was passed because many Americans thought that there were too many Eastern and Southern European immigrants entering the country. They felt that these kinds of people (Poles, Italians, Greeks, etc) were not likely to become Americanized. They didn't like their religion (Jewish and Catholic) and they worried about their political beliefs. Because of this, many Americans pushed to enact laws like the Immigration Act of 1924. This law was meant to restrict the number of Eastern and Southern Europeans and to ensure that immigration would come from Northern and Western European countries.