Either because he thinks that the media cycle will make his whistle-blowing irrelevant, because he's worthless after he discloses everything, or because then he'll no longer be a celebrity. Take your pick depending on your opinion.

Right now he has been noticed by almost every politician in the world. Prime ministers and presidents mention him and everyone is wondering what will happen to him. Perhaps he wants to keep the focus on him until he is granted asylum.

It makes sense to produce a slow stream of information so that he doesn't become old news. If the media starts ignoring him the politicians may not care as much.