Because then Greenwald and Poitras and the Guardian would be under threat of imprisonment to reveal their source, and would be the target of White House retaliation for revealing classified information.
Faced with these threats, no publisher would go with a single anonymous source (unless, of course, that source is "an unnamed administration official"). They would be far more easily convinced by the White House / Pentagon to keep the documents under wraps, or destroy them. That's why Wikileaks found a niche to fill.
Also because coming forward gives him some protection from retaliation -- if Snowden remained anonymous and they found out who he is, he'd probably just be assassinated or, worse, locked in a dungeon somewhere for eternity.
All in all, I think Snowden did a fairly competent job for someone faced with an ethical dilemma: break the law to reveal a greater crime, or obey the law and conceal a greater crime. But his refusal to face the consequences of his own crime undermines his ethical position; even Manning did this. He mostly did the right thing up until he accepted Russian asylum. He needed to lawyer up and agree to turn himself in on condition that he receive a fair trial in an objective court, and monitored probation until such time as such a court could be found. If ever...