Religious fundamentalism.
Unlike like most areas in the Northeast and Midwest, Evangelical church membership is higher than "Mainline" church attendance in the Southeast.
I wouldn't say that's the main cause although it is one of many. I mean states like Idaho, Alaska, and Montana are pretty conservative overall, but they have some of the lowest church attendance/importance of religion in daily life rates in the country.
Likewise states in the Northeast like MA and RI have large numbers of Catholic voters, but are still solidly Democratic.