In the airline industry, Atlanta is a world leader by multiple measures. Hartsfield-Jackson International Airport is the world's busiest airport, and Atlanta is home to Delta Air Lines, the world's largest airline through its merger with Northwest. Delta also operates the world's largest single-airline hub at Hartsfield-Jackson.

But for all of that, Atlanta still isn't served by the airline that carries the most domestic passengers in the United States --- Southwest, which is also the nation's largest low-cost carrier.

Southwest's absence became more conspicuous when the Dallas-based carrier tried to buy Denver-based Frontier Airlines earlier this month. The deal that would have poised Southwest to finally enter Atlanta by taking over Frontier's Denver-Atlanta route. But Southwest lost out to Republic Airways in the bankruptcy auction for Frontier, and Atlanta travelers were left wondering what might have been --- just as they were when previous rumors of Southwest's arrival proved false.

For much of its history, Southwest avoided large hub airports where costs were higher and delays more frequent. By the time it started serving them, the stiff competition between two strong carriers in Atlanta --- Delta and AirTran Airways --- left little room for a third competitor to wedge its way in and grow.

Moreover, Southwest has "had bigger fish to fry" as it expanded into the northeast and elsewhere, said KKC Aviation Consulting partner Stuart Klaskin.

Delta, along with merger partner Northwest Airlines and Delta Connection carriers, handles about 75 percent of traffic at Hartsfield-Jackson, while AirTran handles about 18 percent, based on May 2009 passenger data from the airport.