Two things to consider here,
1.) the long memory of southerners dating back to the Republican carpetbaggers after the civil war, as you say, it took over a hundred years to finally let that dog lay,
2.) the greater influence of the Bible on southerners, more powerful to a true southerner than even his long held animosity towards the perceived enemy of the south and the “common people,” the “northern” carpetbagging Republican party.
Number two won out, thank God.
So, why are American Blacks such left wingers? They're practically mirror reflections of white Southerners in their Biblicism. Yet not only do they vote for far left candidates, their churches seem to produce only far leftists so far as politics is concerned.