Why do so many people in the North think that all of the people in the South are idiots?

I'm 13, and I have lived in the south for most of my life. My mom is from New York, and my dad is from Georgia. One example of this "prejudice" (since it seems extreme to call it that), occurred when I visited my cousin in New York. She seemed shocked to realize that I read and enjoyed both the Harry Potter, and His Dark Materials trilogy. Apparently she had umed that they were "against my religion". I'm an atheist. I know that that is a very minor example, but I see this kind of thing in movies and on TV often. Why do some people judge everybody in the South as being ignorant fundamentalist bigots?