As it is surprising it is logical - they are competing with RyanAir, EasyJet and partly Norwegian on the same market - the mentioned bigger players have brand new planes and average age of Jet2 fleet is astonishing 22 years !
They are running one of the the oldest 738 fleets (almost 15 years in average) + fleet of 733 + 757 both over 25 years.
I believe the time for some fleet change simply came.
You seem to have changed your s543 tune from 7 days ago in a post in the Airbus orders thread Quote - It is hardly imaginable Allegiant ordering a new plane - complete nonsense, similar to JET2 no way !
And here is the very very..... Unexpected sight of JET2s first ever NEW aircraft