That'll be the same reason Americans don't elect a Juris Doctor who was President of the Harvard Law Review, right?
Plain and simple:
Obama got elected because he is a great speaker and he's charismatic. Qualities much closer to being a popular actor or religious figure got him elected rather than his intellect.
It didn't hurt that people were tired of the previous party (this is a common pattern).
Obama was the first President of the HLS in yeeears not to publish. He's a very, very smart man but calling him an intellectual might be pushing it. His popular works are not brimming with ideas.