In the early seasons an extended opening sequence plays sometimes. It seems to have been abandoned by the third season with only occasional uses after that. I really liked this version and I wonder why they stopped using it?

I'm guessing it had to do with the length of the script--more dialog, shorter opener. I agree, I like it--a really nice theme by Henry Mancini.

They probably went to the shorter version to allow an extra commercial to be squeezed in.

I'm guessing it had to do with the length of the script--more dialog, shorter opener. I agree, I like it--a really nice theme by Henry Mancini.
I really love how Newhart used the same theme(music and video) for the entire run of the series. The only thing that changed was a couple of the actors' names. It adds some nice stability to the show. So many other shows feel they have to revamp their openings for subsequent seasons. I am glad Newhart didn't do that.