Ultimately I think it comes down to cost. As airlines predominantly compete on ticket price, they'll do whatever it takes to reduce that (outsource their logistics, operations, flight planning, etc.) Hell, Ryanair in Europe make their pilots pay for their uniform. All about getting the fare as low as possible, consequences be damned.
Except they can't easily reduce labor costs because of the unions. Southwest is not unionized yet almost every employee there seems happy and friendly. Flying on Southwest is almost fun and they have low ticket costs. Southwest knows how to properly hedge against fuel increases and as such they're one of the few profitable airlines around.
