Because his most powerful weapon is controlling the public discourse.
If he goes entirely silent then the government will step in to fill the void:
"His girlfriend's a stripper, he's a hacker terrorist, his neighbors say he's weird, he might be a soviet spy, he did it all for the attention, he's not an American."
If he speaks out on a daily basis the news media will eventually grow tired of the story and move on leaving him to be a 3rd page story when he's arrested.
His best option right now is to stay in front of the news for as long as possible until some solution is worked out. In the meantime staying positive with the American people will do more good for his eventual trial.
To apply pressure on the US government regarding the revocation of his passport making it impossible for him to reach Ecuador or where ever, of course. He's stuck in Moscow due to his passport being revoked and Putin's basically told him that he's not welcome there. He's in a tight spot.
You'll notice he never once mentioned the documents or their contents in this message. He's not "making the story about himself" - he's using the media as a tool to apply pressure to the forces aligned against him in the hopes of securing his safety.
It's a plea for public support. By raising Obama's profile as an oppressor, he hopes to turn public sentiment against the sitting government.
As someone else said, remaining silent lets his opposition control the discourse. You can see this clearly in the frenzy of stories in the media about Snowden's stripper girlfriend and the "IS HE A COMMIE SPY?" headlines. Providing a counterbalance makes it harder for the government propaganda machine to sway public opinion against him and turn him into Just Another Terrorist.
I don't think it reads like a plea for public support, if it was intended as such then it isn't worded well. It reads like a bunch of other wikileaks press releases that are generally political statements about the importance of transparency. The problem is that if he wants public sympathy he needs to avoid coming across as having political motives and if he does want to make accusations then he must consider the possibility that they will backfire and cost him public support.
A 'counterbalance' can often instead be a distraction which is what I fear happened here.
We know he has been in Moscow for a long time! There isn't anything new here at all. As others have pointed out the statement used British phrases instead of american ones so it appears this was drafted by wikileaks instead of snowden, which isn't very reassuring.
Because, like anyone in that situation, he's trying to find a home. Unfortunately, for him to receive justice he needs to be in the spotlight because politicians won't make the right choice unless confronted by their constituents.
If he chose silence, he'd be infinitely detained, and would receive no justice.
Justice for Snowden is a lifetime in jail, according to basically every government in the world.
That's his fundamental problem, after all. If any of the countries he intends to flee to were to have someone leak the same kind of documents that Snowden leaked about the US, that person would be prosecuted to the fullest extent of the laws of that country.
So when you talk about justice, I don't think you want Snowden to receive justice, because that's a small cell for the rest of his life, or worse.
Your reasoning presumes that countries of the world apply the same standard of 'justice' to foreign spies that they do to their own. Clearly this has proven to not be the true for China/HK and Russia.
Because history repeats itself, and you should learn from history.
Joan of Arc stood for France, in the Hundred Year's War. The story was not about her, it was about the country, the people. But yet, her narrative, the ideas she had, her martyrdom, etc all had a profound impact on the events of history, and lead the changing of the tides of the war.
A war is not fought on a martyr's backs. Sometimes, it takes 22 more years for the job to be finished. This doesn't mean putting someone up as an example isn't warranted, nor is it dilute or make the narrative of the people any less powerful.
How Snowden is treated sets a precedent for future U.S. dissidents of any kind. You might not care for or about him, but what happens to him sure as hell matters.
