Because it's a key transportation hub.
Started as the 'Terminus' (right in the city's original name) of the Western and Atlantic Railroad, and continued to expand due to Hartsfield-Jackson, which has historically been the world's largest airline hub.
I know ATL is one of the largest airline hub since it was the base of Delta, but that should be after ATL became a mega metro?
It had the geographic advantage. It was where the north-south railroads going from NY southwards terminated or 'turned' East-West around the Appalachians into the southern interior. Before the civil war it was a major rail hub, but this location ensured the city would rise from the ashes and become an even bigger rail hub after industry came southwards.
This post implies though that Atlanta specifically is an obvious place for a city, as opposed to a historically lucky place at the junction and termini of rail lines. The Appalachians, although they made building railroads difficult, did not make them impossible.
In fact, the most obviously convenient link between the large NE port cities and the south is through the Cumberland Gap, and yet Knoxville never became a great American city.
Atlanta in fact has a somewhat crummy location; it's not along a major river (the Chattahoochee goes a good bit to the north and west) and unlike most major cities in the region it's not on the fall line either. It's only the magic of railroads and their erasure of distance that overcame that and made Atlanta a major city. It could just as easily have been Birmingham or Augusta or Athens or Macon, etc.
As late as 1950, the greater Atlanta area only has ~110,000 people on greater Birmingham.
Apparently the issue was a rail line to the Midwest, not the NE.
https://www.quora.com/Why-is-Atlanta-where-it-is
