Obama won, among other reasons, because traditional Republican turnout was down approximately 10%. McCain does not inspire Republicans. He is not a candidate of ideas. His political claim to fame is “reaching across the aisle”. A candidate of ideas would have others reaching out to him.
That's assuming 'traditional Republicans' were sitting out. I doubt that's the case. In some cases, they probably simply voted for 0bama, or they voted but 0bama's numbers are so great.