Delta Air Lines stranded an estimated half million passengers in its service meltdown last week, leaving many sleeping on floors of airports, frustrated and angry at their inability to complete their trips, unable to find their bags or get answers from the airline. But the fact is no matter how mad they are, most of them will return to the airline in the future. To paraphrase the old song, for most travelers, breaking up with an airline is hard to do. The reality of the American air travel system is that powerful forces keep passengers flying the same carriers, even after service meltdowns like Delta just suffered. America’s four largest airlines — American, United, Southwest and Delta — control more than 70% of US air travel. For passengers who live near one of the hubs where Delta controls most of the flights, such as Atlanta, Detroit, Minneapolis or Salt Lake City, choices are even more limited. And many customers have frequent flyer accounts and branded credit cards that feed into those accounts, binding them to one airline or another. Story has more.<br/>