The simple answer that not many people vote.

 But the simple answer isn't what we're after, (because if we were after a a simple answer this would be the real answer is that people don't vote for smarts, they vote for someone who they would trust in their home with their children.

So at first glance this may not seem to answer the question at hand, but it gives the answer that Smart people do run for office, they just don't get voted in.