There was a programming shift in the early 1970s from targeting an older audience to a younger, more urban demographic by CBS - but this was more due to their seeing their ratings stranglehold slipping to ABC (and to a lesser degree NBC) due to ABC’s heavy programming toward younger audiences.
And simply because a show had a urban setting didn’t mean that it had a smaller audience, either. For example, “All in the Family” had HUGE ratings in rural America.

Finally, the so-called rural purge wasn’t absolute by any means either, as there were still rural-themed shows like Gunsmoke and The Waltons anchoring their scheduling days.