A couple of things. Although he doesn't look it, other media have reported him as being 71-years old! He might just be slowing down. Other media are also reporting that the contract banned him from doing other things. It seems kind of strange that after 25+ years, the new contract prohibits this whereas the previous ones didn't. And then, didn't Fox originally say that this was the last year of The Simpsons? Harry might have started planning other projects to fill his time. I think he's still doing a weekly PBS show and works on documentaries and comedy releases. But certainly, prohibiting him from doing other projects would have been a sore point for someone as busy as Harry.