This does seem a strange omission from the XC stopping pattern.
Chepstow has no direct trains to Bristol (15/20 miles away), few to Birmingham, it gets a relatively raw deal really (considering the commuter potential).
It's a relatively big place for such a low amount of service, I don't think it could ever have direct trains to Bristol. Well, unless they were to reverse at Seven Tunmel Jct: but I'm not
Sure the capacity of the seven tunnel Would allow it. The XC services would see the place connected a lot better with major cities In the UK