Why Atlanta became the mega metro in the south?

[Oryxslayer]

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.

Interesting. But I guess during the time when railroad was important, there should be many other cities serving as hubs? Why ATL was the most important one?

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.