So, I guess we have Bill Murray to thank!

Ramis: Murray blocked ‘Ghostbusters 3′
Who you gonna call? If you’re director Harold Ramis, not Bill Murray.
In the new issue of Script Magazine, Ramis tells journalist David Cohen that he cites Bill Murray as one of the reasons why a “Ghostbusters 3” never materialized.
“Bill Murray wouldn’t appear in it. I think he didn’t want to see a sequel made so he became kind of obstructionist about it,” Ramis is quoted as saying.
In the draft script entitled “Ghostbusters: Hellbent” written by Dan Aykroyd (based on a story by Aykroyd and co-star Ramis), the Ghostbusters team once again to fight spirits and demons. Due to an overpopulation problem in hell, the Devil evicts diabolical souls back into the living world. The Ghostbusters have to then use their high-tech gadgets to send the souls back where they belong.
“Analyze That,” Ramis’ sequel to 1999 comedy hit “Analyze This” starring Robert De Niro and Billy Crystal, hits theatres on December 6th.