Khan is not some untranslatable villain. He's Star Trek's most iconic villain from what is still perceived as the best film in the franchise. Why not promote that?
Perhaps because they already felt that tons of people would see the film anyway based on the success of the previous one, that they decided to keep Khan a secret to actually, you know, have some fun with it

Take Star Wars for example. If Disney decided to revive Darth Vader for episode 7. Do you think they would keep that in their pocket till the film was released or promote the hell out the fact that Darth Vader is returning?
That's a good question. I suppose it would be the same situation as above: Would they feel that because it's a new SW movie, that that would be enough to get millions of asses in theater seats, or would they need a carrot on a stick to lure them there?