Look at Hunger Games, that was barely promoted until maybe two weeks before release, then all the fangirls gave it the biggest midnight opening ever, all the word of mouth and great reviews made it into one of the most successful films of the year.
It's easy for a film like Avengers to reveal all it's money shots in all the trailers and tv spots, because there are so many eye catching, humour filled, crowd drawing moments to show people, it's an easy film to market. TDK was even easier, all you needed to do was show Heath Ledger chew up scenery for 30 seconds and you have yourself a hit, It isn't that easy for TDKR, where it's two main draws will be critical reception and the TDK/Batman brand, the best time for this film to really show its cards will be in the month of July which allows WB to really make their event film feel like one. Show it too early and you are just burning people out on footage or having it swallowed up by Avengers, Spider-man, GI joe, Prometheus, Men in Black etc considering this is probably the least crowd friendly film of the lot.