I don't know why they're waiting, but if I had to say honestly, probably for publicity. They've recently had some major funding problems and they likely realized that such a big story, drawn out, would get them more media attention (as it has:
WikiLeaks to release video of civilians, journalists being murdered in airstrike | Raw Story ) and subsequently more funding. Sad to admit the possibility of it, but probably true.
It also just came to me the possibility that all of this government interference could just be a story to gain popularity as well. We'll see I guess.
Edit: from the comments i'm seeing online it seems everyone else is saying the same thing about their motive for waiting: to build up media suspense. While it might seem like a jack move for this kind of organization, they really do need funding and if they want to exist at all in the future (and they have done great things) they simply need more money.