Linville's explanation involved several factors. Hotlips wasn't his ally and confidante anymore, he felt he had done all he could as the character, and felt that Frank was only being used as a continual butt of the joke. Sounds like he was simply burnt out with Frank.
He may very well have said that, however, that attitude was a luxury brought about by his being woo'ed away with the promise of starring in his own series. If he (or Stevenson for that matter) hadn't had other opportunities dangled in front of them then they'd have tried to stay.