The other is a September 29, 1985 Associated Press article about Peter Scolari, who after being a guest star in season 2 was brought on as a regular in season 3 to replace Kampmann.
The producers decided to make the switch when it became apparent that Kampmann's character had nowhere to go. The character was supposed to antagonize Newhart, but he had become too strong.
"When you have an obnoxious character you can go one way, as Danny DeVito did on 'Taxi,' and become a character you love to hate," said Scolari. "Or you can have a character who's obnoxious and doesn't realize it, as I do with Michael. This year Michael will be a lot more human. It's to avoid the same trap that Steven fell into, and to keep in step with the rest of the show."
Taking those things together it seems like the writers thought Kirk was too depressing and unlikable of a character (getting him married was supposed to make him more relatable, but it actually made him worse) and Newhart didn't think he made a good comic foil.