There's that and disgusted conservatives who stayed home. Obama won because the RNC which pulls the strings continually decides it's more important to have a candidate they favor than to win the election. As long as they control the party, they don't need to control the election.
As far as changing his mind now, this is the time anyone smarter than Obama (almost everyone) has a way out. He could say that with new information he received at the summit he has decided that attacking Assad now would have too much risk for WMD getting to terrorists.
The 2008 GOP primary was stolen with vote fraud...
McCain would have been worse than Obama, it would have put a Republican label on asinine progressivism.