You seem to be forgetting that the AA route planner gives you the typical road journey time - as opposed to the journey time when someone wants to be in central London for 9am, or leave at 5 or 6pm. I'm afraid that the typical traffic volumes on the M40 are unlikely to permit you to manage a 150-minute journey at those times. You also seem to be forgetting the not exactly minor matter of parking a car in central London, if someone was going into London from Worcester for the day.
No I hadn't forgotten those things and there also is the opportunity to work on the train, as pointed out by the other poster and then there is price. I would think that nobody commutes ('nine to five' jobs) with
private transport into central London, from anywhere, except perhaps chauffeur-driven big cheeses with underground car-parks at their offices, but I think leisure travellers, at least half of pax I believe, could have a different set of issues to deal with in making a choice. I think that the GWR commercial consideration is that neither Worcester, nor Oxford, even together, could generate enough traffic to justify frequent non-stoppers. It's almost a captive market for commuting, except on price, (see competing coach services from Oxford), whereas from Bristol, GWR obviously think that such
will be justified all through the day from December. OK, strictly speaking only from and to Bristol Parkway, but BRI (Temple Meads) in 80 minutes with the one stop is not shabby. I presume that GWR has judged that such just knocks any competition into a cocked hat and it still has its 'slow' service (!) via Bath to provide connections at Reading. The Badminton route is a Bath and Chippenham bypass. That is not possible on the Worcester route due to the lack of paths from Didcot, as explained. So it's quantity of the available market and its competitors, that is deciding who gets these non-stop paths, I conclude, with Bristol snaffling nearly the whole lot!