According to Frank Oz, he no longer has a relationship with Disney, which owns the rights to The Muppets and its many characters, or the people at Sesame Street.
His explanation indicates that he wants to stay true to the creative process and type of characters he feels are right for the programs, and he has spoken out about Disney’s handling of The Muppets in the past.
Oz is an incredibly creative individual and in addition to giving life (and voice) to a wide variety of characters, he also has had a hand in the writing and directing processes for projects.