Marky,
What you should remember is that shows like Happy days still have reruns playing a fair amount of time on TV, so the dvd sets would be a bit harder to sell. Where as the Muppet show hasn't been on TV for awhile and people are really wanting it. Season one sold very well, so I'm sure that their are planning on releasing season two.
Their are a lot of shows that are still going stong, so the dvd realeases take more time. For example, Anamanics and Pinky and the Brain dvds sold very well, and their already planning on realeasing vol. two before christmas, where as, say, the Simpsons seasons dvds take longer time due to the show still being on the air, where as Pinky and the Brain and such hasn't been seen on tv in some time. Another great example is "Family Guy", when that show was supposed to be "cancelled", and they realsed the vol. one set, it sold though the roof and vol. two was on the way, however when the show got pick up for new episodes and adlut swim airing the reruns every night, now it's easy to find used dvds of vol one and two, and the vol three, although still sold very well, didn't do as well as the first two vols and the fourth vol hasn't been rushed to release.
Know what I'm saying?