What matters is not how smart the candidate is. What matters is his ability to get enough people to like him and trust him that they vote him into office. Say what you want about Obama and Bill Clinton, in public they came across as likable guys.
This is Trump's advantage -- despite his wealth, he comes across as somebody who I think I would enjoy having a few beers with. I think Walker would be a better President, but Trump is somebody I would enjoy an evening with.
There is another advantage that Trump enjoys: he is seen by women as an alpha male. Compare and contrast:
Jeb Bush and wife:
Trump, wife, and son:
They might not consciously think it, but one impression that both men and women will have is "If he can attract and retain a wife like her, he must be a special man".