Why it is taking so long to come out between releases is anyone's guess. Maybe it's the economy? Then again, we're talking about DISNEY here.
Disney are very keen to use the Muppets, so if it were EASY to get these series sets out, they would have done it by now.
It's not Disney's fault that the Muppet Show has so many songs that need clearance and fees paid to use them and these things take TIME to negotiate and complete.
Disney may be powerful but they're not invincible - they can't just do whatever they want because they're Disney, they have to get in line and pay music rights like everyone else.
It sucks we have to wait, but that's the nature of the show.