Just talking out of my butt, I'd guess the ULCC problems are due in large part to the legacies adopting basic economy, super saver, etc fares to compete with ULCC. I'm sure the ULCC managers gooned some stuff up too, that's pretty par for the course for almost all airline managers.

Even SWA (not a ULCC) is moving to a more legacy-ish paradigm (assigned seats).