A lot of unsuccessful TV shows have been released on DVD. Among them:
*Get a Life (two single-disc volumes of four episodes each, but I have heard that a complete first season set is in the works)
*Greg the Bunny (was cancled after 10 episodes, all 13 produced episodes are included in one set)
*The Tick (cancled after one season, complete series is available on DVD)
*The Critic (was cancled after two seasosn, and both seasons were released in one set; Of course, reruns of this show were included on Comedye Central)
*Wacky Races (though I'm not sure if this was cancled or not)
*Clerks: The Animated Series (cancled after only two episodes, all six produced episodes have been released)
And some unsuccessful shows are coming to DVD in the near future, including:
*Police Squad (six epsiodes were made, all had previously been available on video, and this series inspired the Naked Gun movies)
*The Weird Al Show (all 13 episodes are coming August 15)
Also, every episode of The StoryTeller and Greek Myths have been released on DVD, and more recently, they were packaged into a two-pack. I'm not sure if you'd call these successful or not. So a lot of unsuccessful shows are available on DVD, many in complete form.
Also, even if The Jim Henson Hour wasn't successful, people know who the Muppets are, so if some recognisable Muppets like Kermit were featured on the cover, people might buy it. After all, The Muppet Show: Season One set was successful, and it's rumored that plans for a Muppets Tonight set are planned for after the fifth season release (though Muppets Tonight is only slightly more successful). I feel the same way about Little Muppet Monsters (just put out a DVD set and include images of the animated adult Muppetsalongside the monsters).