The South and Party Switching:

[ElectionsGuy]

A lot of deep south white dems in state legislatures (and even nationally, like Gene Taylor) represented districts that were won by national Republicans by wide margins. The thing is, usually the way you vote at the national level represents ideology more than any other election, and state legislators need to be representative of their districts ideology. That's pretty much the reason I think, but someone (Miles, Griffin?) could tell me I'm wrong. As for why it hasn't happened as much in places like Kentucky, Tennessee, Arkansas? Those places were slower to adapt to more Republican voting. All of them voted for Clinton (except SC, which you referred to). The KDP still has lots of power, and Arkansas just shed its Democratic control last year. I think in those places people are more able to see beyond just party, they are less "stiff" (inelastic) in their voting and mindset. This has generally been true throughout history too (Democrats consistently got >80% in every race in the deep south, but not in the upper south).