Randall Stephens, editor of the Journal of Southern Religion, says it has a lot to do with the South’s cultural homogeneity for most of its history.
I suspect too that the fact that the South remained agrarian (and comparatively poorer) longer than the North has something to do with it.
The way you answer this question will in part depend on whether or not you think religion is good or bad. If the former, you may find virtue in Southerners accounting for their residual religiosity; if the latter, you’ll chalk it up to character defects (e.g., “poor, uneducated and easy to command.”) Can we please try to avoid value judgments about religion, or Southerners, as we try to figure out what cultural and sociological factors have made Southerners on balance more open to religious identification than other Americans? (Note that this doesn’t mean that they’re more moral, only that they more readily identify with religion).