I don't think it's a case of them being disappointed with Man of Steel's box office exactly. It's on track to make $675-700 million worldwide. But they don't want to settle for "just" $700 million. They want to ensure that the next one makes one point whatever billion dollars, and to get to that level superhero franchises usually need a special extra ingredient. For the Batman films it was Heath Ledger's Joker. For the Iron Man films it was the "Avengers effect". They're banking on Batman being that special ingredient for the Superman franchise.