This came out of the blue but makes a lot of sense in retrospect.
God of War - I do agree that if someone was to hold a press conference mentioning the fact, it would be like making a big deal out of nothing. But in this case at least, it's not like he held out a megaphone and yelled it out to people on a sidewalk.
He was interviewed; and in the process of that interview where he was speaking about gay bullying, he mentioned he was gay, almost as an aside ("As a gay man...") to make a point.
It's the media that takes it out of hand and blows it up into a big reveal. But they do that with every kind of story related to celebrity gossip (i.e. Lohan and Hilton) so I don't see that as a particular singling out either - it's just the way tabloid culture works unfortunately.
In terms of why actors only tell after they've made a name...well, I think gyro summed things up.