I can think of a couple of reasons. Kerry's dad wasn't paying anyone to do his work for him? Kerry was putting in a lot of time leading the anti-war movement while Georgie was cheerleading and hazing skulls and bones pledges? Kerry signed up for a challenging curriculum while W took classes based on answer availability in the test file at his frat house? As a wise man once said, you can get all A's and still flunk life.
Or maybe, just maybe, the aw-shucks country boy dumba$$ regular guy thing is just an act...maybe, perhaps, he didn't just bumble into this mess but did it on purpose. Granted...crazy theory! I mean, who has really benefitted from this fine mess in Iraq?