A CBS TV revamp, circa 1971. They axed rural shows - The Beverly Hillbillies, Petticoat Junction, Green Acres, Hee Haw, and Andy Griffith's Mayberry RFD - and replaced them with city-based shows. The cancelled shows weren't unpopular - they just didn't attract the desired demographic. The purge made room for shows starring Bob Newhart and Mary Tyler Moore, et al (including one starring Griffith in an academic setting), and for All in the Family.