Seats are rubbish , would be my best guess at the main reason.
On the 319s on Thameslink we had decent, comfy soft seats. Race along at 100mph in perfect comfort. They sent them up north, largely in one piece and again, Liverpool to Manchester in perfect comfort.
Then Northern weave their magic on the seating, and it's an endurance feat.
I've been on one of the new Northern trains, and although the seats aren't as bad as the ones they soiled the 319s with, they're not far off and they definitely do transmit ride quality to posterior with little loss in power...
The 319s have the exact same seats they arrived with
Some of them still have FCC moquette post ‘refurb’...