If 9/11 is somehow prevented (and there are no other successfully attacks by al-Qaeda on U.S. soil before the 2004 Presidential election), then who exactly would have won the 2004 Democratic nomination and the 2004 U.S. Presidential election in this scenario? Al Gore? Hillary Clinton? Someone else?
Bush would still have been the incumbent running for re-election. Any incumbent has an advantage. As a result, the big candidates in the opposing party usually avoid running against them and they’re challenged by the second tier.