What happened? I think fundamentals were trumped by mechanics and, to a lesser extent, by demographics.
The Obama campaign strategists—and congratulations to them, by the way—argued that they would win by organizing and turning out the vote in the key states that would determine the outcome of the election. They had no illusions that they could expand the president’s appeal beyond the 53% of the popular vote of the 365/359 electoral votes they won in 2008; on the contrary, they conceded Indiana’s 11 electoral votes and the single electoral vote of the Nebraska 2nd congressional district even before the campaign started. They didn’t contest the 15 electoral votes of North Carolina very much after holding the 2012 Democratic National Convention there; they concentrated in their pre-convention negative anti-Romney advertising and in their organizational efforts on a three-state firewall of the next three states in order of Obama 2008 percentage, Florida (29 electoral votes), Ohio (18) and Virginia (13).
After Obama’s disastrous performance, and Romney’s sparkling performance, in the October 3 debate, it looked like the firewall was crumbling. But it seems to have held.