First, the "confession" -- whosever they are trying to elicit -- has more value if given out of paranoia.
Think about it this way: you have a brief glimpse into something wrong that happened, but you don't know the full story. Let's say you know 10% of the full story.
If you come forward with the evidence, the accused party need only acknowledge or refute the 10% that you've divulged.
However if you make them think you know a whole lot more than 10%, you might end up pressuring them to give a confession that will reveal a much larger amount of the wrong deed.
Second, pressuring a government to admit wrongdoing is better PR for Wikileaks than disclosing leaked information. Wikileaks is treading on delicate territory already with their mission statement. When classified information becomes whistleblower-deserving information is a debatable topic. Because of this, Wikileaks has to manage their PR very carefully, lest they be viewed as the enemy-to-all, friend-to-none. By pressuring organizations to admit wrongdoing rather than publicly accusing them of it, it's a win-win: the information gets out, and WikiLeaks doesn't have to be in the press again for leaking "stolen" information.