I feel like it was the feud with Harold Ramis that made Murray so coy about Ghostbusters for all that time. Unfortunately that was just too sad and ugly for either of them or anyone associated with them to talk about publicly. But reading about the feud now, and how it didn't end until right before Ramis died, it seems clear this would've been the biggest impediment to Murray doing another Ghostbusters.
https://www.vulture.com/2018/06/how-har ... -feud.html
Ramis and Bill Murray had been friends and collaborators since the ’70s and worked on Caddyshack, Meatballs, and Ghostbusters together, but their friendship fell apart on the set of Groundhog Day. Star Murray and director Ramis had such intense creative disagreements that one day Ramis grabbed Murray by the shirt collar and threw him against a wall. After that, Murray didn’t speak to Ramis for over 20 years. Stiel says her father “did his best to be diplomatic about the whole thing” and “tried not to take it personally” since Murray had been going through some personal problems at the time, but to little avail. The falling out got under his skin — Stiel says that her dad told her he felt alternately “heartbroken, confused, and yet unsurprised by the rejection.” Just before Ramis died, Murray showed up unannounced at his old friend’s house at 7 a.m. with a box of doughnuts and a police escort. Ramis had pretty much lost his ability to speak by that point, so Murray did most of the talking. They didn’t rehash the events from the set of Groundhog Day, choosing instead to hang out for a couple of hours, laugh, and make amends.
