As to the delay, Nolan again cites the difficulty studios have had with third chapters of successful franchises: "Godfather 3", "Superman 3" "Star Trek 3"… I could go on forever. Here's the quote from the Nolan himself instead:
See that? The man has our best interests at heart – and perhaps his own. Nothing can fuck up a career like following the second highest-grossing film of all time with something along the lines of "Spiderman 3", "Shrek the Third" or… stop me anytime.
Nolan went on to tell Bowles that he has been writing notes and rough outlines for a third installment, but has yet to come up with anything he feels is film worthy:
Help Nolan stick his landing by checking out the entire interview here