Perhaps they believe that gaining the necessary amount of public attention and support requires these things to be revealed slowly and to consume many news cycles. I would argue that the pace things have been proceeding at has been perfect for those who generally don't pay attention to get a sense of how important an issue this is.
If this had all been released at once, it would have been much easier to sweep under the rug, and those who generally reflexively defend their political parties might not have had to really confront the issue. They could be proceeding this way secure a better chance of victory.
This is why I think Snowden is such a sharp guy, because of the fact that he has been releasing documents slowly and methodically as opposed to a mega-dump wikileaks style disclosure. The basic problem is that a large share of the American public (such as my mom) get their information from exclusively one source. In my mother's case it's CNN, where there hasn't been very much coverage of this other than the odd report here and there. For instance looking at CNN right now the main headline is an article about this most recent school shooting, and the NSA revelations are a small print link jumbled in with lots of other unrelated articles.
I think you're confusing Snowden and Greenwald here.
The way I read it so far (please do correct me if I'm wrong) is that Greenwald received a very large dump of information from Snowden and that Greenwald is dictating the pace and selection of the releases.
If this had all been released at once, it would have been much easier to sweep under the rug, and those who generally reflexively defend their political parties might not have had to really confront the issue. They could be proceeding this way secure a better chance of victory.