1. The economy just sucked. It was too badly broken for Obama to fix it, or his solutions (targeting banks early on but not forcing them to help ordinary people more) were not sufficient. In retrospect, how could a president possibly win re-election with unemployment this high and with a stream of forecasts about anemic growth over the next year?
2. A miscalculation about Romney. I’ve sketched this idea before — that by not painting Mitt Romney as a flip-flopper, and instead casting him as an out-of-touch plutocrat, the Obama campaign lost the ability to call out his late-in-the-campaign transfiguration into William Weld. Also, voters simply don’t trust presidential flip-flopper aspirants. The Mitt Romney voters saw in the debate wasn’t the guy who they’d heard about.
3. Obama fatigue. As much as they liked him personally, they look back at his presidency and feel a struggle. It’s hard to look back at the last four years and smile; his presidency, through maybe no fault of his own, really, was necessary to get the country back on track, but he had to do a lot of things that were very unpopular, and because he governed from principle, and not politics, he paid a price for it.