I'm not sure of the year, I'm guessing it was the late noughties that Manchester got a regular hourly service to Bristol with some extensions to Plymouth and Paignton, and trains from the north east got an hourly service to Reading. Prior to that the Manchester trains mostly went to Reading / Poole and North East trains to Bristol and beyond.
When these changes were made it was probably deemed too costly to send the whole depot route learning to Bristol, especially as they would only be able to get one trip in each day, probably in the same way Derby drivers didn't sign to Reading.
Derby drivers can take Manchester trains from Birmingham to Bristol and Manchester drivers can take Newcastle trains from Birmingham to Reading, so there's no real benefit to the company for them to sign both routes.