Jim Henson only saved a small amount of Sam And Friends episodes. back when the show was made, most shows were broadcast live and a lot of shows were not saved, mostly due to the high cost of film and because many people didn't think that people would want to see the same television program more than once. Also, Sam And Friends was a local show, so it would have been more expensive.
So, Jim Henson pretty much only saved episodes where he did something that he really wanted to remember. I don't know how many episodes were saved. Since this show was five minutes long, it may not be likely to be seen in reruns (especially since reruns of the show were never syndicated). I think the number of episodes that are saved could fill an entire DVD (and even a video).
Another reason why having the show on DVD is because even though Kermit was in this show, Henson still owns the rights to it. Kermit was a major character on the show, and he seems to appear on most of the existing clips that have been shown to the public. I read in the Muppet Central review of the new DVD release of Emmit Otters Jug-Band Christmas that Disney actually did refuse to allow Hit! to keep Kermits scenes in the special. Ever since the DVD was announced it had been said that Hit! just didn't feel it was neccessary to make a deal to include Kermit, yet the review says that Disney didn't want kermit included at any price. I wouldn't be surprised if Disney would refuse to let henson leave Kermits scenes in a release of Sam And Friends.