It's Colin Powell syndrome. Would YOU want to run for President of the US? Most people wouldn't want to drag themselves and their families through having every detail of their entire life turned into political campaign ads.
Therefore, we usually aren't drawing from out best people. Or, our best people that are willing to run aren't willing to do what it takes to win - see above, taking the "centrist" position.
No candidate that I would fully endorse and be 100% comfortable with could ever win. Many Americans are the same.