Overconfidence Helps, But Why Underconfidence

Technology Review has a piece on why overconfidence (the idea one is an above-average driver, etc.) is evoluntarily sound.

In fact, overconfidence is actually advantageous on average, because it boosts ambition, resolve, morale, and persistence. In other words, overconfidence is the best way to maximize benefits over costs when risks are uncertain.

So why are some people lacking in confidence (as in social relations), why do that evolve?