They could be refusing to run it for a number of reasons, lack of platforms at Rugby, issues with staff, something to do with ticket acceptance or Avanti. I DMed them on X (aka Twitter) to see why they aren't but they still haven't responded. I'll post an update if they do.
Not long got an official reply from London Northwestern Railway so I'll summarise it here.
First of all the decision for them to terminate at Northampton rather than carrying on towards Rugby was an operational decision on their end as they stated they did not want to effect services from Northampton towards Euston, however they did say that the majority of their crews were based out of Birmingham and Coventry.
Relating to another post that I saw here a day or so ago the WCML is not Bi-Directional and services that are running into Coventry have been placed on speed restrictions.
Finally the few Avanti services running that way are either heading towards Scotland (mainly Edinburgh from my own research) or into Oxley in Wolverhampton. I'll leave the full unedited message I got below.
Hi Gio,
Firstly, I do apologise for the disruption over the last few days die to the landslip. The decision to terminate at Northampton, rather than Rugby, is for operational reasons. Our services change crew at Northampton, so one set of crew work the train from Birmingham and far as Northampton and another set of crew work the Northampton to Euston part. By terminating at Northampton, this protects the service south of Northampton and avoids issues relating to crew working the Rugby - Northampton service, as the majority of these crews are based out of Coventry and Birmingham
Secondly, the West Coast Mainline is not bi-directional working. Given the line speed and safety implication with Network Rail staff working on the London-bound (up) line, the decision was made for everyone's safety to only run at a reduced speed towards Coventry when it is safe to do so. Whilst theoretically we could run a train to Birmingham, this service would then be stuck and unable to head back. A limited number of Avanti's have gone through this way, but they are mostly headed to either Scotland or the depot in Wolverhampton so they won't get stuck trying to head back.
Hopefully, the line should be re-open soon, but if I can be of any further assistance, please do get in touch. ^JR