Well because they are hits on Bush the man. That's what Kerry's campaign has been about all along. The problem is that Kerry's own credibility has taken a hit as well so he's arguing that he's the man of greater integrity from a rather weak base.
Tactically, the Kerry campaign/CBS/NY Times/Boston Globe attack on Bush's national guard service this week was a positive for the Kerry campaign simply because it is soaking up attention that would otherwise be focused on tomorrow's 3rd anniversary of 9/11. The timing was certainly not a coincidence. Kerry needed some interruption between the GOP convention and this weekend.
Actually, if the source of these do ents is the Kerry campaign or they knew they were fakes, well, it's over. Which of course would be rather stupid because the Bush lived a wild life and a privileged one as a young man has already been established in the national consciousness. They didn't need to risk so much political capital for so little.
Also, if it does become clear that Rather went public with do ents of dubious origin then whatever credibility as a "journalist" he had is gone forever. Where the Kerry campaign ends and CBS News begins is not clear.