Yesterday does not make any sense at all. Obama got 10 million less votes than 2008. If we had done slightly better than McCain we would have won. How did we lose voters? Something is rotten here...
The answer to that question is as obvious as the nose on your face. You saw a lot of it right here on FR during the primaries and the run-up to the election. The difference in number of voters for McCain and Romney are those who either stayed home or didn't vote the top of the ticket, only down-ticket candidates. You saw it expressed many times on FR. It was the "Romney-is-as-bad-as-(or worse than)-than Obama" meme. There were many people who said, don't vote for Romney, vote for down-ticket candidates. As you correctly note, that resulted in fewer votes for Romney than McCain, whereas if we had simply matched McCain's vote, we would have won. Those non-voters for Romney DID make a difference - they gave us another four years of Obama, and probably the destruction of the Republic.
Unpleasant as it is to face, I am speaking the truth, although I realize than doing so in this context on FR runs me the risk of being zotted or banned. At this point, I don't care. I'm about done with it all after this election. Only disgust and a sense of loss (of the Republic bequeathed to us by the Founders) remains.