I’m unclear about why he is taking so long to dump it all.
Maybe he wants all of it looked at. If it all comes out, some points will be overlooked as less important.
Him starting out with 2009 G20 made me think he’s going to go by date, not shock, order.
Americans have short memories. Its better to slowly leak stuff to keep the pressure on and expose the government’s lies.
Plus his safety is not guaranteed. There is no reason to use up all his ammo yet. Better to aim carefully and keep landing direct hits.
Others have made suggestions why, but it's pretty clear in his answer regarding Wikileaks, if you read his between the lines.
He believes that he is only disclosing things he believes are in the public interest. He is specifically withholding any information about "US operations against legitimate military targets".
Wikileaks dumped a bunch of unredacted information because someone screwed up. That wasn't their intent (the group, not necessarily Assange). Snowden appears to want to prevent this from occurring again.
He’s gaming the press cycle.
The press loses attention. The American public can’t keep their attention on one thing for a very long time without being reminded.
If you really want to effect change, then you “pulse” this information into the press, to keep the story high up on the press’ agenda every night.
He’s played this very well. He understands that there’s about a seven day news cycle now, and he’s pulsing information into the press about every four to five days.