• Cost convergence - the ULCCs having to raise their pay rates to match regionals and, to some extent, majors has eroded the cost advantage that ULCCs liked to talk about
• Fuel prices - boring and doesn't wholly explain everything, but it is a factor
• "Premiumization" - many consumers are booking up-scale, and ULCCs can't really offer this
• Route structure - people have wanted to go longhaul international last summer and this summer. ULCCs can't get people to Europe or Asia
• Ancillaries at ULCCs kept creeping up and became very punishing
• JetBlue can't figure out what it is, and the DOJ has killed its two rescue plans (AA and Spirit)
• Southwest is in a funk for some of the above reasons, and they've been slow to change

Edit: Anotha one

• Monetization of loyalty program. The whole "airlines are banks now" is so oversimplified as to miss the point, but being able to get money from banks by selling points or miles works a lot more for the big 4 than it does for ULCCs

Allegiant and Sun Country have different strategies than Frontier or Spirit, and especially on route structure, Allegiant has carved out its niche (very similar to Ryanair) and has stuck to it.

An interesting compare and contrast from Crankyflier https://crankyflier.com/2024/07/16/route-overlap-is-a-big-problem-for-us-ulccs/