As the country exited COVID, mainline carriers began to really push their basic and barebones fares which directly took away business from ULCCs like Spirit. Spirit never really leveraged their credit cards effectively to help subsidize the cost of seats.

ULCCs thrive when they can maximize their time in the sky across as many different flights as possible. Filling up the day with many different flights allows them to rake in checked-baggage fees, reserved seat fees etc more often than longer mainline carrier flights. All of this went out the window when a large portion of their A320 fleet was grounded because of the P&W engine issues. This absolutely killed their utilization percentage.

Spirit died way more because of business management than it did because customers didn't choose on price.