Because they were the only reports available, having been (at best) vetted by the White House first.

Kerry's vote was only one of a potential 100 in the Senate. He did not -- could not -- make policy at the time, but only vote yes or no, up or down on the issue presented.

So you might say Kerry was duped, just like every American and the rest of the world, without the chance to know otherwise. The absolutely very best face you could put on Bush and his advisors is that they were duped, too.

If they weren't duped, they were either idiots, or fanatics that refused to listen to sources that disagreed with their pre-judgement of the issue.

Which do you want to vote for -- a dupe, an idiot, or a fanatic? You have no other choices.