It seems to be confirmed that Disney was the source of the cancellation, that was apparently announced along with the cancellation. but the reason for doing so is far less clear. Fear of leaks regarding the movie is certainly a possibility, but that possibility exists regardless of whether or not Hayden Christensen is in the movie. McDiarmid's own role is still shrouded in a lot of mystery and is almost certainly central to the story, so anything that gets accidentally revealed from him would be a big spoiler.