All sorts of reasons (who knows why?)
The truth is the four-year degree requirement is a relic from the old days when the airlines got their pilots almost exclusively from the military.
Just officer types, you know?
So if you populate one organization with one type of people then they tend to recruit people just like themselves.
But, as years passed, the military couldn't keep up with the demand of the airlines so more and more civilian pilots were recruited but the degree was *almost* always a requirement.
The rationalization was/is the degree proves good study habits; the ability to focus on a long term goal and succeed; etc.
Right now, in my humble opinion, I think the degree is just there as another reason to narrow the field of applicants. Just like requiring 1000 twin turbine PIC *or* 500 twin turbine PIC *or* holding a 737 type even if you don't have any operational experience.
They're all bogus requirements but it's a super-competitive market right now and the airlines can pretty much get exactly what they want.
Jetblue *prefers* Airbus experience. Guess what? They're getting it.
Fly safe.