It is not really that simple.
In their defense they started buying AirBus aircraft a few years back and could not have known what was to take place.
With that said, low-cost carriers thrive for several reasons. Probably the most important one though is flying with a standardized fleet (one type) of aircraft. Southwest does the same thing. It greatly improves flight schedualing, simplifies training and makes maintenance workers masters of that particular aircraft. Now compare that to an airline like American that uses 10 different aircraft.
So I can't blame them for not going against the phylosophy that has helped to make them so successful. Can you?