At its five most critical hubs—Atlanta, Detroit, Minneapolis, New York, and Salt Lake City—Delta has less exposure to lower-cost rival Southwest Airlines Inc. American and United, meanwhile, have far more capacity overlaps with Southwest in their hubs, particularly Chicago, Dallas, Denver, Houston, and the San Francisco Bay area.

That's it in a nutshell. Also, Delta's hubs generally don't compete with cheaper to operate out of secondary airports (MDW, HOU, DAL, OAK, etc.). In the case of ATL, the operational costs are rock bottom to begin with. From a business standpoint, ATL is the best airline hub in the US by far.