Others here basically summed it up: Gazoo was seen by many as just too big a departure from the show's normal goings-on. the basic premise of a "modern Stone Age family" aping The Honeymooners was baroque enough, add to that the gimmick characters of the Gruesomes, who were Gothic monster types existing millions of years before the Gothic movement had been invented was enough of a stretch, but a magical green alien from the future was just too much and he really didn't fit in with the rest of the Flintstones universe. Plus, as Shawn mentioned, Gazoo didn't really help to prolong the series; the show just ended after his episodes. We didn't even get an on-camera resolution to his plot of having to perform good deeds so he'd be allowed to return to his home planet Zetox (sp?), so he gained the reputation of being the show's "jump the shark" moment on top of that.