I think I read somewhere, long ago when it was either declared a rumor or canceled film, that production halted due to Jerry Nelson's death and Kevin Clash's departure, but I doubt those factors would have had an effect on the movie. After all, Matt Vogel had been trained to take over Jerry's characters and has done a great job as The Count (and the only other Jerry Nelson character who has heavily been used in recent years was Mr. Johnson), and Ryan Dillon has also proven himself well as Elmo so far.