I don't know, it doesn't make any sense if it does, the Civil War was 150 years ago, and it didn't have anything to do with racism. I think it has more to do with closed-minded intolerant bigots, who have adopted a stereotype of Southerners, and refuse to let it go.
The US civil war had nothing to do with racism? Wasn't slavery a prime cause/excuse for it?
The fact that it was 150 years ago doesn't affect people's opinions.
We haven't fought the French since the mid C19th yet we still have the animosity. The Battle of Glen Coe was centuries ago, yet the Scots still go on about it.
Also you have the modern history of institutional racism in the south, with lynchings and the KKK et al.
You're right, it'd be a bigotted person who tarred the whole of the south with the same brush but you can also see why that impression comes about.