Jacob B., it may have been because he had become typecast after the Martian role, and had trouble finding work for a while after it was over. His N.Y. Times obituary mentioned the typecasting angle, and how he knew that would happen, but took the role anyway for financial reasons. This happens to so many actors because of T.V. roles which are too lucrative to turn down. Fred Gwynne is another who comes to mind. After Herman Munster, one of my all-time favorites, he too disappeared for quite a while.

I also enjoyed Walston's performance in the remake version of "Of Mice and Men".

Clifteevee