I think because people who were basically undecided expected to see something that they didn't get - some excitement, some set of programs or plans or visions or anything at all but the bland "not-Bush" that came across. Taking the anti-Bush hatred out of the Democratic convention would be like taking the stuffing out of a sausage - what you have left is going to be a tough sale.
He was everything to every one. He can make 280 million citizens happy.