Clearly, it's a combination of a lot of things: scaling back of both Muppet and human characters, doing away with spoofs and parodies . . . in a sense, it almost feels like a deja vu of when Jon Stone said of the ATC era that the show was being "dumbed down," which resulted in him being fired. One of the things about SST was that they always made it a show that parents could watch with their kids, however, the research is turning up parents don't watch with kids anymore, hence the end of parodies - because kids don't understand them - and celebrities - because kids don't know who they are; research is also showing that most kids shows have smaller casts with only a few main characters, hence why they're scaling back on both Muppet and human characters. But the thing of it is, all of that has been part of what made SST the show that it is, so I can see all of that playing a contributing factor in Joey's decision to walk away.
EDIT: Ziffeled twice.