Purely speculative article, but the delivery timelines for potentially better airplanes than the E2 (like the CSeries/A220) are growing... With the long order backlogs, it would take 4-5 years to get a new NEO or 737-MAX ordered today. Doesn’t Spirit want to maintain its high growth rate? The article makes some good points:

https://finance.yahoo.com/news/why-spirit-airlines-might-order-154000628.html