He's being held responsible for eight years of Republican failure
Personally I think that there really isn't any daylight between McCain and Bush on any subject that matters, but even if there were, he's the only Republican available to act as the target for years of frustration and outrage. Many people feel that the system has betrayed us by allowing Bush to go unpunished for very real high crimes and misdemeanors, while a few years ago Clinton was impeached for a trivial sexual dalliance. Hard not to sympathize with that feeling of betrayal. McCain's put himself at the center of the bullseye, and for better or worse, all of the anger so many feel toward Republicans gets focused on him.