[mmbt0ne]

Among other reasons:

Delta has 50+ vice presidents, all of whom make a pretty hefty salary for doing dick basically. AirTran functions just as well with less than 10% as many.

Gas costs. Even a full plane of passengers with ticket prices where they are now will not pay for the upkeep and maintenance of a plane. Let alone the salaries of those flying/servicing them.

AirTran's deal with Boeing. Only this spring has AirTran started to pay for the brand new planes they got a few years ago. This means, while they were able to charge those ridiculously low fares earlier, it was because they were paying $0 in plane costs other than the minimal upkeep that a new plane requires. As their flights beame more popular, other airlines had to lower their prices. Lower their prices to the point that they weren't making money.

Also, another thing with Boeing. They give airlines $1M discounts to take their old planes out into a desert and cut them in half, rather than sell them. Delta, AA, United, etc. all uesd to sell old airlines to other carriers, but now that portion of income, no matter how small, has been minimized even more.

There's a lot more to it, but that should get you started.