Low cost carrier. Think Ryanair. Competition from the rest of the market and bad management put them in a bad position, with the most recent war causing unsustainable fuel issues.
Other airlines may be able to double/triple their prices in the short term. Spirit's customers may simply choose to not fly.
https://www.npr.org/2026/05/02/nx-s1-5807933/spirit-airlines... describes this in more detail.