The reasons for such an effect are, in my mind, several, and the primary must be that Snowden has thought to secure evidence (such as it is) about the agencies' actions.
The second one is the way that he went about disclosing the evidence.
It all started with the revelation of the order issued by the secret Foreign Intelligence Surveillance Court (FISC) that compels Verizon to share metadata on all the phone calls its users make with the NSA, and Americans were shocked not only that it happens, but that the fact was expressly kept secret by the government.
Then, a day later, another "bomb": the NSA has direct access to Google, Facebook, Apple and other big Internet companies' servers. This time, the rest of the world joined in the outrage.
It was later discovered that perhaps that particular statement wasn't entirely correct, and that the companies apparently send the wanted data to the government, but the effect is the same. And again, the gagging order: the companies can't talk about it, confirm or deny it.
This all happened at the end of the week, and the public was left to stew and has been given time to mull about the information. Then, Monday: the whistleblower reveals himself in a video, and he's calm and eloquent - critics say only at first glance - doesn't over dramatize, doesn't seem unreasonably paranoid, looks like a young professional that you could meet at any respectable American firm. As first impressions go, it's not bad.
Then, the day after, a new revelation: Boundless Informant - the U.S. is spying "everyone, everywhere". Couple it with the previous disclosure of U.S. president Obama ordering national security and intelligence officials to draw up overseas target list for cyber-attacks, and the rest of the world is shocked one more time, especially the EU and Germany (the UK a little less, it seems to me).
I don't know whether it was The Guardian's own decision to release these disclosures in such a perfectly timed and slowly escalating manner, or whether it was Snowden's idea. Nevertheless, it was pure genius: the short breaks were just enough for the various parts of the U.S. government to react, but not enough to coordinate the reactions, making it seem like they are, indeed, hiding a lot.
Snowden's timely and dragged out disclosures have also served to keep the issue fresh in the minds of the public, and each new outrage has effectively incorporated the previous one.
It has also helped that he contacted established, respected reporters. Wikileaks might have published all the slides of the infamous NSA slideshow that The Guardian and The Washington Post didn't, but its public image (and that of its founder Julian Assange) is not stellar.
The U.S. administration is now in full damage mitigation mode, but Snowden already did partially achieve his stated goal: jumpstarting an actual, public debate about the merits and drawbacks of the U.S. government surveillance of its citizens.
Legislators, intelligence officers, cybersecurity experts and privacy advocates have taken sides - some considering Snowden a hero, others a traitor.
Snowden support and smear campaigns are also in full swing. He has left the hotel in which he originally stayed in Hong Kong and has "disappeared" fro a couple of days, resurfacing today to throw another "curve ball" to the U.S. government, saying that they have been hacking civilian targets in Hong Kong and China for years.
And the explosive revelations, according to The Guardian's Glenn Greenwald, are not over yet. I don't know if they can top what we have already heard, but I guess we'll see soon enough.