And why isn't the resource available ? Lockdown proper ended months ago.
Simple answer is almost certainly training.
Put simply if a given number of people retire (or leave for other reasons) then you need that same number of people to replace them.
Certainly for my neck of the woods retirements (etc) have continued unabated, in fact some have been brought forward, whilst replacement hasn’t been able to keep up anywhere close, partly due to recruitment issues, but mainly due to the almost total lack of training which occurred for a large part of the year, and even now is constrained.
Just for current staff there’s a training backlog, with many routine training interventions not having occurred since March. So you have more people needing training for whatever reason, and less capacity to deliver it due to social distancing (etc).
Something has to give, and choices have had to be made as to where the axe falls. You can’t run something with resources which just aren’t there. Naturally it probably doesn’t help that Northern don’t seem to have been in a brilliant position before this kicked off.
If someone draws up a service plan which requires 50 driver duties at a depot to run it, then you have to be pretty confident that there are the numbers to be able to cover those 50 duties every day. If that’s not going to happen then quite simply cancellations are going to occur, and the nature of crewing is such that it’s very difficult to match uncovered duties with less important services. Furthermore, the evil nature of crew-related cancellations is that the effects spread out to affect more than the segment for which there’s no driver (eg incoming driver needs to stable train so is now late for his own next train). I’m not saying Northern couldn’t have provided timings more useful, but as a rule you’re better with having them attempt to run a service they think they have the capability to deliver, than taking your chances with something they don’t, and having multiple messes throughout every day according to where the uncovered duties fall.