ETA: Also, yeah, QT's bang on about the reasons for Smith leaving (it's his choice, they didn't let him go). The Doctor is a role that dominates a CV, and actors can find themselves struggling to get out of the shadow. Even Ecclestone did and he left after one season. Tennant doesn't appear to care because he's a massive fanboy, but it seems to factor large into the thinking of others, both new and old.
Which may be where Capaldi differs. Not only is he as big a fanboy as Tennant, but he's an actor who's already had a good long career and one properly, properly iconic role in his backlog, so he may not feel that pressure to leave. He may be around for a while. But we'll see.
This post has been edited by polishgenius: 05 August 2013 - 12:34 PM