Years and years ago excuses were made for the legacy investments of the big airlines, but bailout after bailout later (explicit and through bankruptcy), it's clear that the people who run most of the airlines are just bad at it (or good, depending on whether you think their incentives really are to maintain a successful profitable business or to pump, extract value, declare bankruptcy or merge, repeat. )
Southwest cracked the code, including a bit of luck here and there, and their practices aren't mysterious.
