This was supposed to be Clinton's year. A run by Obama this time would have put him in the best position for a 2012 or 2016 campaign.
Think how many Senators run in each election cycle and how few get the nomination and you can see how unlikely it was that Obama would become the nominee his first time out.
Having been nominated, Obama's in over his head. He doesn't have the record, or the skills, or the team to take him the rest of the way.