Why Atlanta became the mega metro in the south?

[Del Tachi]

Railroads are a good answer for why Atlanta first became a big city back in the Reconstruction Era, but they don't explain why ATL grew so much bigger than other regional centers like Birmingham, Nashville or Raleigh in the second-half of the 20th century (as Sol mentioned.)

The founding mythology of Atlanta is that it's a post-war city that has always been uniquely progressive, enterprising, and open to outside (i.e., Yankee) influence.  Locals will proudly tell you the reason Atlanta's so successful is because they're "too busy to hate."  Take that as you may. 

I'd also mention the 1996 Olympics and the associated redevelopment of Downtown and Midtown Atlanta as very transformative.  The games and their associated development activity soundly established Atlanta as the most valuable commercial hub in the Southeast, and that activity continued to fuel a lot of redevelopment/gentrification throughout the 2000s and 2010s.