[ue]

A mix of two things

1 - slow to urbanize until after WWII, meaning as Boston, Philly, and NYC industrialized and became behemoths, Charleston and Savannah weren't and the South was still predominantly agrarian until air conditioning became commonplace.

2 - the major rail lines (and later highways) by-passed Charleston and Savannah, going for cities like Atlanta and Raleigh, which was crucial in the air conditioned age, when the South began to urbanized (and suburbanize).