Why do startups fail?

Startups are risky ventures: more than two-thirds of new companies close. Their investors realize that only a small part of the investment will pay for itself. The reasons seem simple: bad ideas, poor implementation, and poor management. However, very promising projects that investors believe in have a market and potential and are run by smart people also fail. Why does this happen? I have identified three factors. Hunters, what do you say? 1) Good idea, bad mates. 2) Premature launches. 3) False alarms.