How is anything Snowden released hard evidence? Every single thing Snowden released could have easily been faked. Hell, those powerpoints look fake. Who thought an intelligence agency would use something straight out of the world of TPS reporting and cookie-cutter presentation templates? Note, I'm not saying they're fake, but it would be trivial for someone to make this stuff if they wanted. So the argument that there's this overwhelming proof that the public can't ignore seems like weak sauce to me. The public isn't terribly interested in hard proof (or have the time/expertise to weigh claims), historically. If they were, we'd probably be a society of hard atheists scoffing at anything that can't be trivially proven in the lab.
The real issue here is why certain things go viral and others don't. Snowden's fleeing was fairly dramatic and I'm assuming that helped get the ball rolling. The timed release aspect of his disclosures are fairly clever too. Frankly, I think Assange cut a path for him by normalizing this type of leak and the journalist/news system was ready for it. In the past, this system was too busy avoiding these issues, or worse, being a mouthpiece for the current administration (See the NYTimes' disgraceful Iraq pre-war reporting, repeating Bush admin propaganda without question.)
Without the Assange/Manning path, the previous whistleblowers only received marginalized exposure. Now news managers realize how profitable leaks are especially with the White House not reciprocating with the 'blocking of access.' Obama's more liberal direction probably guarantees more leaks in his terms. We'll see how things are with the next POTUS. I suspect a GOP POTUS won't be as forgiving as the Bush administration has shown (Valerie Plame/Wilson affair for example).