It was wildly successful because it was arguably predatory. American consumers were either unfamiliar with the unbundling concept or the market segment was too stupid to pay attention. That's why ancillary fees were a majority percentage of the revenue.

When you add up the cost of traveling with luggage, it was hardly low cost. You might save $20/ticket over the legacies. Which was not worth the hassle in case of delays/cancellations as Spirit didn't have the capacity/frequency/structure to reaccommodate passengers quickly.

People wised up and now either travel with no luggage or they travel with someone else. Or pay an extra $20 flying basic economy with a "more reputable" brand.

Lastly, have you looked at the pay of the likes of Ryan Air in Europe? It's significantly less than than their "national main brand" counterparts and they skirt labor laws with the way their bases are structured. When Spirit's labor cost creeped up, it was the last straw, it was no longer a low cost airline, just a crappy one.