Looking at some Toughpigs posts, I read that Craig Shemin said that the reason most Jim Henson Hour characters weren't included was because the rights were so murky that it'd likely never get a home video release. So I guess the lack of those characters was because the fans might not be able to see those characters (well they could search YouTube.....).
I wonder what's so "murky" about the rights. According to the 2004 documents which showed up online last year, Disney did get the rights to the Muppet portions of The Jim Henson Hour, and the JHH characters were listed among various copyrights that transferred to Disney. I wonder if it's due to Jim Henson's introductions including clips from later in the show (though I know they recorded alternate intros, which were used in reruns when MuppeTelevision was shown on its own). I wonder if it's because of any of the monitor images. They all look like they were recorded just to be on the monitors to me, but Muppet Wiki says that some of the images are from existing productions, like Sam and Friends. Maybe it's because of the music rights, though the show doesn't feature as many numbers as the average episode of The Muppet Show.
But then if leaving out characters is because they might not ever get released on video, this gives hope that the rest of The Muppet Show and Muppets Tonight will be released eventually. The book has the Snowman and Turkey from A Muppet Family Christmas, I wonder if that provides hope that that special might return to video (though there are the out-of-print releases...).
I also read that some characters were left out because the productions were so old that only us hardcore fans would really care. I wonder if it was really "too old" (after all, the Sam and Friends characters are in there) or more "too obscure". The book does have Splurge and Rufus, but not King Goshposh or Featherstone.
I feel like the book should have had a section listing all the Muppet Show and Muppets Tonight episodes, for the sake of the first appearances. Episodes are listed by numbers, with no guest stars listed. Casual fans wouldn't know the episodes by number.
In recent years, there have been nonfiction books on Sesame Street and Jim Henson which had sections on other recommended nonfiction books, and even acknowledgements of Muppet Wiki and other fansites. But this book doesn't have such acknowledgements.