Petticoat Junction and Green Acres were axed as well. The network execs decided that wholesome shows with country bumpkins was passe to a generation of newly socially and sexually liberated millions.
Add the Beverly Hillbillies.
Those series had long, long runs. They just ran out of material. In their latter years, the scripts for episodes became silly and downright stupid. Even many of the original cast had retired, died or gone on to other programs. That is the problem with long running programs.
I do agree that CBS management had a change in emphasis and wanted more sophisticated programs.