Prior to the 1990s, with a few exceptions, most networks when promoting their shows wouldn't advertise what would properly turn out to be the final episode of the season of a show except for usually indicating that its a new episode. Sitcoms in particular were subject to this as well as most non-serial programming. I feel this was done to persuade viewers to keep watching into reruns, and many local TV sections didn't mention that episode as being the last until the fall. In the 1970s and '80s, when there was very little summer replacement programming, reruns of course were the norm at the Big Three and PBS, so perhaps "hiding" the fact that future episodes will be repeats won't alienate their audiences?