Class
Trump had appeal to evangelical, nominally evangelical and non-evangelical WWC voters in the deep South while Cruz's Evangelical base was more high end. This meant it dominated the activist base completely enough to win or compete in closed and/or Caucus states, but Trump could swamp him in Southern primaries, especially open ones.
It also helped that the earliest and likely one of the most hostile states (South Carolina) had an incredibly divided field allowing Trump to win with only getting into the 30's and was likewise open.
On the other hand he did rather well in states like TN and AL, rural GA, and areas close to the Appalachian mountains, particular those with a hollowed out feel economically (The rust belt doesn't stop at the Mason-Dixon Line hence Trump killing it in Buchanan Co, VA), large populations of white working class voters, or both. These voters have been streaming into the GOP since the 1990's thanks to Clinton's scandals, Bush's cultural conservatism and the continued shift of the Democratic Party to the left on the social, environmental and gun issues and in those states that were open, Trump could augment those with even more of the same group that remained as Democrats or Independents.
Also this:
http://politics.blog.ajc.com/2015/12/26/that-time-when-donald-trump-saved-a-georgia-farm/Discovering this article the day after Christmas on AAD, is what made me realize that Trump could beat Cruz in the South.
