It probably has a lot to due with the fact that all the southern Democrats switched to the Republican Party because the northern liberals were making them too uppity about civil rights. But there had to be something in the Republican Party that attracted them in the first place; which probably came down to a conservative/progressive divide. To the southerners progress wasn't good if it meant giving minorities equal rights.

Thus, the Republicans inherited the conservative Bible belt. It was only a matter of time before the conservative Christians overtook the almost libertarian-like traditions of the Republican Party. Instead of the small-government Republicans switching to the Democratic Party, many of them stayed due to party loyalty (and in a process of brain washing are duped into supporting a big-government and big-business party), while some went independent.