Maybe because few people actually do the full journey from Waterloo to Exeter, whereas on GWR many people are going the full length trips.
Hence my different views for the Bristols/Cardiffs and the Westcountry service. I
suspect you'd find that the latter has the largest proportion of people doing the full journey (or near to it) than any other service of that kind of length. It really does call for something more than a trolley - to me a DB-style Bistrowagen would be an excellent fit for it. It really isn't like most other UK IC services. It's much more comparable to the Chieftain or KX-Aberdeen direct services, or even the Caledonian Sleeper minus the kip.
The shorter services are fine with a trolley provided it's reliably there, of course.
What's also worthy of note is that if you're on an Advance it's less likely you have chance to stop off for refreshments, because your ticket doesn't allow it. So on a walk-up I might break that journey up somewhere like Plymouth to have a meal, but if I'm on an Advance I can't...