Did other TOCs block them?
View as extractive, they would stop at peterbourgh if the could. Why the can't stop to set down is a mystery. At least then the cover the hull - peterborough Market and gain no revenue out of the London - peterbourgh pot.
They'd still be taking money for Grantham, Retford and Doncaster though going the other way.
...Unfortunately open access can't be considered for the greater good of a clockface timetable!
That said I don't know if GC passengers would want the extra stops and slower journey time. I am sure the management would not mind the ORCATs!
That would come down to how busy the services are between Sunderland and York and between Bradford Interchange and Doncaster. If most of GCs passengers are picked up on the mainline I'd suggest the extra stops wouldn't make much difference (to the majority of passengers) as they've always got the East Coast services.
If Grand Central and Hull all ran; Kings Cross, Stevenage, Peterborough, Grantham, Newark North Gate, Retford, Doncaster...
then either stops to Hull, stops to Sunderland, or stops to Bradford, that'd fill holes for the stations served. The 180s would be busy probably all the time, too. They certainly aren't at the moment.
What I don't understand is why HT and GC don't work together and share paths? I mean, if they wanted they could timetable a GC from Bradford to arrive in Doncaster and join to a 180 from Hull. They can then go onto London together, and the same the other way around. If EC didn't want GC to call at Grantham etc then only open the doors on the HT.
There are quite a lot of technical issues which mean that some 180s don't join to others - apparently there are two different types? These issues would have to be dealt with before this could even be considered.
How come Hull Trains don't stop at Peterborough or Newark North Gate? Surely with such a move they'd inrease patronage and thus revenue. Did other TOCs block them?