Why did some Americans blame immigrants for the nation's problem?

Because Americans have some issues.

Explanation:

America is a big melting pot of people from different places, religions, races, you get the idea. Some Americans, I'm not going to go into detail on who, believe that only natural born citizens or people that have English descent have certain rights. While some of the problems that Americans think immigrants bring are true, such as immigrants bringing some diseases, much of this is made up, as some humans can't accept the fact that some people practice different beliefs or look a little different.