Atlanta is the hub for two major domestic carriers- Delta(also international in this case) and Airtran. The “busiest” tag comes from the number of passengers passing thro the airport. Being a hub, the two airlines carry passengers from all over first to Atlanta and then put them on connecting flights out to wherever they are eventually headed. That is what causes such a large number of passengers to walk through Hartsfield.
Second as as been said by others many other large cities have more than one airport. If you were to combine the trafiic thro all the the airports for a particular city and then make a comparison, I am not quite sure Atlanta would qualify as the “busiest”.
