Basically, cause they're jealous. I'm sure there have been bad run-ins with the locals, but that happens north and west as well as south. Lots more people are influenced by writers of the south, the ones who dwell on the plantation themes, and stuff like Gone With The Wind. The only thing I can insert here, is if you don't experience it, don't stereotype it.
And my ancestral home, btw, is Mississippi. I've live in Virginia, Louisiana, Florida, Colorado, and am now "at home" in Texas.