Maybe the placement of season three so close to season two set saw a drop in sales? The long wait could be an effort to make up for that.
Possibly. But nothing to warrant a 2 year wait. I think whoever said the music rights were dealt with wasn't 100% accurate. Unless they're still working on certain ones. Really, music rights are the WORST. It protects companies from not getting huge payoffs from denture adhesive companies who mangle the lyrics, even against the original family's wishes.
But what if it's an appearance foul up? Don't they have to pay rights to use all the actors? If it's because of the Star Wars episode, I'd be surprised. Lucas, Disney, and Henson share a good relationship...otherwise we would have never seen the Star Wars muppet set.