Hammering Bush about Osama would have been a fairly risky strategy, because if Kerry went that way and Osama was caught it would kill Kerry's election chances.
Attacking Bush about the concept of the 'war against terror' was a non-starter. It's far to cereberal and subtle a point to be made in American politcs. Worse, the press would oppose losing one of their ready made media issues*.
Jay
*Note, I don't think many involved in the media would intentional oppose Kerry to keep their media issue alive. But there is a big, unconcious bias against playing down anything they can press as a top billing news item. The press would continue to play the 'News you can't afford to miss: *WAR ON TERROR*' bits as their top news items because they get the ratings.