It's the old, old argument: (1) buy a mature a/c fairly cheaply and accept high maintenance costs or (2) finance mega-expensive new a/c that are cheaper to run and maintain, and are more reliable.
If your business model depends on starting and building up a large number of new routes you need spare capacity to expand your network. That means that, initially at least, you risk having seats sitting on the ground, especially in the off season. Old, cheap seats are more sensible for this. When your network is mature, you know your capacity requirements and need to drive down costs, so newer a/c make sense. On that basis, expect more 73s and 75s which have been around a bit, rather than shiny new Airbusses or NG Boeings. I, for one, would rather that Jet2 continued to thrive and expand rather than it all go pear-shaped with millions owed to the manufacturers....