Another factor (as referenced previously) is capacity. With the FAA pressuring ORD and other metro airports to reduce capacity, they're going to lag in terms of passenger traffic.

Also, Atlanta, AFAIK, has few if any day/time restrictions on operations.

And lastly, our lovely Southern climate probably also helps us along. With little or no snow or ice to contend with and thunderstorms usually just a transitory phenomenon, I'd imagine that the weather gives us an edge.