I honestly have not heard what Trace and Frank had to say. Can someone bring me up to speed or point me toward a source?
The main source I recall was a podcast where Frank was the guest (with Mary Jo I think?), though he's done quite a few so I can't find the one I'm thinking of. There's also been quotes and general rumor mongering on Reddit since the reboot started, some from folks who claimed to speak with them at live shows.
The TL;DR was Joel shutting down a business that non-Rifftrax MST alumn were invested in without warning, and not managing the subsequent financial and creative offers in a way the old talent liked.
As I understand the timeline:
Trace in particular was heavily invested in Cinematic Titanic. To the point he was apparently shipping the Blu-Rays from his house (again, so folks on Reddit said). The whole CT crew, and Trace in particular, were blindsided by Joel's decision to disband CT to restart MST; he hadn't discussed it with them or warned them. However, once that ball was rolling Trace and Frank et al assumed they'd be returning to their original roles; a continuation, not sequel/reboot. (This I recall was from the podcast with Frank.)
Once the reboot was up and running, it was always Joel's intent to use an all-new cast, so he offered the old cast bit parts. This seems to be the major sticking point, as Frank and/or Trace called the offer they received "insulting." Frank also seemed annoyed that Patton was playing TV Frank's son, again without any input from him. (This, I recall, was a tweet.)
Again, a lot of this is from random comments made in passing that folks reported hearing at shows etc, plus the aforementioned podcast.
Edit: here's a link to a Reddit comment from someone who says he asked about it at a live show.