Remaining with one's ancestral political party after that party has made itself the party of filth and the rejection of every principal of universal morality is a strange type of "conservatism."
BTW, American Blacks are as religious and (probably) Fundamentalist as any white Southerners. How in the blazes they vote the way (and have the leadership) they do is something we will probably never know.
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.