It appears that the reason Obama won was that many conservatives, including readers of this site, disliked Romney so much that they preferred to have Obamacare, preferred to have the contraceptive mandate, preferred to have the destruction of the private health insurance industry and of the private medical care industry leading to outright socialized medicine, preferred to have an annual trillion dollar increase in the deficit, preferred to have a president allied with and actively facilitating the spread of jihadist regimes in the Mideast, than to have Romney as president. In short, if what the commenters are saying is true, it appears that the most terrible event in the history of this country, the final destruction of the constitutional American Republic, was brought about by conservatives for whom McCain was conservative enough, but Romney was not conservative enough.Why did they stay home? Was it because Romney is a Mormon? Probably not, or the evangelical South would not have gone heavily for Romney.
Some speculation says that many whites are simply fed up with the GOP being too liberal and not adequately conservative. I hope that answer is true...but I really don't know. It sounds too simplistic and self-serving. I suspect that Obama's negative campaign, which started very early in the campaign, was a major factor. It was exacerbated by Romney's late entrance into the fray and a less than effective rebuttal of the Obama slander against him.
See the discussion at View From the Right here.