As noted by others, there are no competing major airports nearby, whereas Chicago, NYC, LA all have multiple airports. Also, DAL was among the first to really push and develop the "hub" system by bringing regional flights into ATL before sending them other places, which increases passenger counts significantly. There is an old saying among residents of places like Birmingham, AL, and other surrounding cities, that if you live there, when you die you have to change planes in Atlanta on your way to heaven (assuming that's where you're heading).

Also, ATL is predominantly a white collar city and many, many S&P 500 type companies have regional headquarters or operations in the area which brings folks in and out on a regular basis.