External forces happen. I've certainly seen businesses shut down for reasons outside of their control. It's just that this happens a minority of the time. Most of the time, businesses shut down either because:

1. High-risk venture. Someone wants to be the next Facebook and takes a 1-in-a-100 gamble at a billion dollars.

2. Stupid idea/business model. Founder is a true believer in some idea which doesn't match reality.

3. Poor execution. It's a good idea and business model, but any of the hundred things which can shut down a business do: technology takes 10x as long to develop and is buggy; founder fails to recruit good people; zero external sales/visibility/marketing ("if you build it they will come" or wildly implausible assumptions about viral or similar); costs get out-of-control; etc. Most founders I know can do all of those reasonably, and one or two brilliantly. It's hard for me to emphasize how important (and hard) it is to be at least competent at all of those since messing up any one will shut down a company.