is the easiest answer. He loved the show and that is clear. He loved being a part of it and is probably more than happy to be a part of anniversary specials and maybe another special now and then.
However, I see no reason for him to do so with his career going pretty well it seems.
The show probably needs him more, but I doubt they realize that. Really, I get the vibe mostly now that those around the show are almost picking the direction of the show out of spite.
Not sure how else some of the choices could be made that are being made at the moment to the changes in cannon.
I would much prefer them to just have David do maybe say 3 specials a year, 90 min long, treat the Dr now as they did Sherlock Holmes!