I imagine by that point, Cosby was probably just ready to give TV a rest, particularly since he'd done the "other" Cosby show on CBS with Phylicia Rashad just a few years before.
I've seen most of Newhart (including the end), but it's been quite a while and it's frustrating that they've only released the first season on DVD. Similarly, the last two seasons of the Bob Newhart Show are m.i.a. and somehow I doubt anyone's pushing for
Bob to be released. There are also a number of other late '80s/early '90s NBC and CBS shows I wish would come out, but it seems less likely every year. And where's my final season of Ned and Stacey, Fox? *scowl*