I'm not an investing expert by any means, so take this with a grain of salt.
From Spirit's 2019 10-K:
Spirit has $1.55B in contractual obligations due for 2020. This does not include the cost of operations, just debt principal, interest, aircraft leases, and aircraft purchase obligations. So while the ~$1.1B in cash/short term investments on hand looks good, it's not enough to satisfy the debt obligations the company has for an entire year even if the airline did not have to consider things like fuel, labor, landing fees, taxes, etc.
To put that in comparison, from SWA's 2019 10-k.
SWA has $3.6B in contractual obligations. However, over $2b of that is aircraft orders (max) which they may not be obligated to pay for considering the aircraft is un-airworthy. Regardless, even if they did take the entire order they have ~$4.0B in cash/short term investments on hand.
More food for thought is how leveraged the aircraft are at each company. While for example Delta is indeed parking aircraft, it's because they have the luxury of having a paid off fleet and can downsize as necessary. I can't say for certain but I would imagine the most of the newer fleet at Spirit would be leased/mortgaged thus requiring payment whether or not an aircraft is generating revenue. Obviously not something great for business if demand falls to nearly 0.
Lastly, I think there is also an investor perception that the government would be more apt to help SWA or the big 3 versus something like Spirit/Frontier... but that's just a wild guess.
That is my best guess as to what I think Wall Street is seeing. In my personal opinion I think SAVE is tremendously undervalued overall though. I think the business model is robust and I also think the type of passengers will be quicker to bounce back versus the business travelers that the big 3 rely on to generate the most revenue. I also believe that even if we ban domestic travel for 30 days, the spool up will go back to normal a lot quicker than most downturns we've seen in the industry based on the fact that this is just a consumer fear issue that hopefully will be resolved if this gets under control.