From what I gather from what I've read over the years...
1)The vintage buses were driven by senior b/o's who drove the fishbowl and Flex New looks when they was everyday buses, retired. Most if not all of the current day B/Os at Quill aren't trained on anything older than a RTS or Orion hybrids.
2)Someone credible said the museum buses get no maintenance, I'm assuming most of them don't even start up which is why we see the same 5 or 6 buses at every event. In the video below (NOT my video) it looks like some of those buses haven't moved in years by how dusty they look, look at 5117 windshield..
3)The TM has a heavy favorism towards the subway museum fleet and the buses are a afterthought.
4)This is just speculation on my part, but Im assuming nobody offered to train the current day Quill B/Os on the museum buses, and if they did, would the B/Os be interested? Keep in mind that the buses older than the RTS do NOT have power steering. It's a lot of work to drive one, also you gotta deal with transit fans which let's be honest, do not have the best reputation lol.
I think these are the reasons we don't see holiday vintage bus run anymore. I could be wrong.