They are, I gather, having to go through all the material first - and there's a lot of it!
They can't just dump it on the net. The most obvious reasons being that it might expose and endanger agents, or reveal important secrets which are not related to spying. There's probably personal information that shouldn't be exposed. All sorts of things. And they can't be sure what the problems might be unto they've actually read through the stuff.
At a guess, Snowden probably laid down some conditions of what could be done with it before he gave it to whoever it was - a list of journalists he felt were trustworthy? I don't know the details.
Biting the hand that feeds IT
