Their whole angle was that they could undercut everyone by betting that enough people are willing to travel miserably if they can save a few dollars. But the bigger airlines were able to adopt this in part and Spirit is now broke.
Planet Money did an episode[1] on People Express, a low-cost airline that sprung up right after deregulation in the 80s. The history of People Express as told in that episode mirrors Spirit. Once the major carriers decided to compete, it sunk the upstart's business.
But at least they made air travel more terrible.