Originally Posted by mineistaken

Because of evil republicans, of course.

That was a joke. And real answer - because of your predetermination. It is a well known thing. In your case:
You see something stupid in the south - damn, people are stupid in the south.
You see something stupid in the north - hmm, he/she is stupid.

You pile all that and your mind get tricked into thinking there are more stupid in the south than in the north just because of your predetermination and because you remember all the situations from there much more.

Just like you do with democrats and republicans:
republican does stupid - hey, look REPUBLICAN did this and that.
democrat does stupid - either nothing or at least not mentioning the word democrat, or whatever, you get the drift.

You can not think cold, logical and objective, that is why you get tricked into false perceptions like that.