You need to look at things like average distance and median distance, do some filtering for most common destinations (example: NYC to LA, San Francisco to Miami, Denver to DC, etc), fuel costs, but also operating costs. Salaries and everything cost much more in the US than they do in Europe.

Cost Per Seat Mile is $0.07 for RyanAir and $0.12 for Spirit, not counting fuel. Spirit hovers around 80% capacity while RyanAir is around 94%.

RyanAir's niche is secondary airports while Spirit was compeating with larger airlines at places like LAX where gate costs are higher.

In 2024 to 2025 there was an engine problem that required Spirit to ground 40% of the fleet to deal with it. Meanwhile they still had to pay for those aircraft with no revenue. This caused a major hit to the financials for a carrier that already runs on thin margins.

I'm sure there's more to it, but these are the larger things I've found.