Martin Campbell who so successfully directed Casino Royale (Filmstalker review) has already stated that he isn't returning to direct Bond 22 because of the workload required to complete it.

In a recent interview he's reiterated that feeling. After responding to a question from Yahoo News about filming in China in the future they said he...

...would not likely be directing the next Bond film despite the global success of Casino Royale.

There's nothing more said on the reasons, but it looks like they haven't changed since his last comment on the subject. However, I think when the producers look at that film, the returns, and the success and think to themselves if Daniel Craig would have been as successful in this Bond without the director it had, we might see him contemplating another offer.

The story was carried through CommanderBond.net and Cinematical.