First of all DOJ required a bunch of divestitures with UNH merger, which if applied to airlines, makes it make even less sense.

Ths issue isn't DOJ allowing or not allowing airline mergers. Its that the airlines worth buying have far too much debt for what value they would bring. Better to just partner and get some benefits that way than a full on debt acquisition. Maybe in a couple years after BK and if Jetblue shrinks considerably it could happen, but for now there is no advantage to any airline buying Jetblue, especially when United will get as many planes in the next 2-3 years as Jetblue's entire fleet.