Because having a through service means that any delays cascade around the network. It's the problem with having very long distance through services.The existing seperate services already cross the WCML/ECML on the flat - so how much does combining them into a single through service increase the performance risk?
If you had a North Wales to Stalybridge service and the train is running 15 minutes late at Ordsall Lane Jn, but there's a service from the direction ofSalford Crescent to the Airport that is due, it's easy: send the on-time train first because the Stalybridge service will simply arrive at Stalybridge a bit late and it won't cause too much of an issue (provided there is a long enough turnaround).
If that becomes a North Wales to Redcar/Hull etc. service then it becomes a very difficult balancing exercise - mess up the Castlefield corridor by holding the Airport service or mess up the TP core and cause ECML delays by holding the Redcar/Hull service?
Out of all the justifications for determining the end point of a service, this surely has to be the worst one known to mankind? Redcar or Saltburn are random endpoints for a service from Manchester but they are still chosen because they are the optimal place to terminate, all things considered.but Stalybridge is a bit of a random endpoint for an inter-regional service
Many other examples are available, e.g. running Worcester services through to Whitlocks End because it has reversing facilities, rather than Shirley, which is where the traffic mostly dries up.
TfW don't really have to be persuaded as such. The DfT and Network Rail ultimately have the powers to impose the timetable they see fit. Of course, if you can get political stakeholders' support, all the better, but again, running through to Leeds to placate politicians is no way to run a railway.A fast service to Leeds might be a nice carrot for TfW
Again, not a reason for combining services unnecessarily.But you wouldn't want to add a TfW service to the TPE core
I'm not trying to be a downer, I'm sure these are all options that were considered at one stage, however there are good reasons why Stalybridge might be used for terminating services that arrive into Manchester Victoria from the west.