1) Because Armitage's source was that infamous White House memo highlighting the Plame-Wilson-Niger connection intended to discredit Wilson, thus making the White House indirectly responsible;
and 2) because of this:
The outing of Armitage does change the contours of the leak case. The initial leaker was not plotting vengeance. He and Powell had not been gung-ho supporters of the war. Yet Bush backers cannot claim the leak was merely an innocent slip. Rove confirmed the classified information to Novak and then leaked it himself as part of an effort to undermine a White House critic. Afterward, the White House falsely insisted that neither Rove nor Libby had been involved in the leak and vowed that anyone who had participated in it would be bounced from the administration. Yet when Isikoff and Newsweek in July 2005 revealed a Matt Cooper email showing that Rove had leaked to Cooper, the White House refused to acknowledge this damning evidence, declined to comment on the case, and did not dismiss Rove. To date, the president has not addressed Rove's role in the leak. It remains a story of ugly and unethical politics, stonewalling, and lies.The problem is that while Armitage was Novak's source, there was lots more leaking going on that really WAS intended to discredit Wilson. Even if Armitage did it first, it doesn't get any of the confirmers off the hook. The rules are clear: neither confirm nor deny. They didn't follow those rules.