Christopher had a public falling out with the creators and writers, and has refused to return for any cameos or specials. The whole story has been covered in the news (google will tell you everything that’s known, along with several rumors) but that’s pretty much all you need to know.

He wasn’t particularly beloved in his episodes, and whether that’s due to writing or acting is a part of the dispute.

It didn’t help his case that his replacement, David Tennant, gave the consensus favorite performance in the role since the revival.