I occasionally watch reruns of Dear John on Antenna TV, just as I occasionally watched it when it first ran on NBC from 1988 to 1992. It got to follow Cheers at first so it did okay in the ratings, but once they moved it, it crashed and burned. But four seasons is about enough for syndication.
Still, why? No one thinks anything of this show. It was based on a British sitcom I've never seen. It's about a support group for divorced people. At its center is John, whose wife walks out at the start of the show (leaving behind a Dear John note, of course).
The main question: why isn't this better? Hirsch had certainly shown what he could do, and creator Bob Ellison had written for The Mary Tyler Moore Show. But on Dear John, the jokes are obvious and the plots ordinary. (Maybe the original was the same, and they successfully copied it.)
TV writer Mel Tolkin once said it's just as hard to write a bad show as a good one. Maybe that's the point. Even with top talent, if the spark isn't there, nothing happens.
PS On my first trip to Los Angeles I went to Paramount to see a show taping. Dear John was available. (I would have preferred Cheers, but that was a tough ticket.) The only thing I remember about the episode was the weak ending, and when I later watched it on TV, they'd reshot it. It was still pretty weak.