Nothing about this is rocket science. The CEO has stated all the reasons publicly.
We need pilots, we need access to an established order book, we need midwest expansion, we need airframes to fill out bos/ewr/fll/lax faster than the organic plan (due to lack of pilots and airframe slots). Spirit is at a significant discount right now, which is what jetblue is exploiting. If you could get all that at a %30-40 discount, why wouldn’t you. This is literally business 101.