Cost, the cheapest AP tickets from Penzance-Bristol at that time of day are on the 0600 service, so people are encouraged to use it for long journeys.
This is the option that most often crops up online for pre-0930 Exeter arrivals and connections beyond if you don't book within a couple of weeks of Advance purchase. It just makes no sense - a two-car Sprinter in the Plymouth / Exeter rush hour being booked out in favour of the long distance trains around it.
Honestly, I know everyone in the country feels the rest of the network has a better train service, but when will Devon and Cornwall have a suitable service? I love travelling by rail, but I would never do a long distance journey these days without driving to Tiverton or Exeter before boarding a train.