Many of the answers already provided are valid. Many startups fail because not enough people want, or will pay for, the product. Others fail because they are unable to generate enough revenue to sustain before they run out of money. In my experience I've also seen startups with great ideas and some traction (evidence of a product-market-message-fit) fail because the entrepreneur couldn't get out of his or her own way. Mindset is key. A willingness to iterate and pivot where necessary is key. A realization that you don't know what you don't know that you don't know as an entrepreneur - and the commitment to fill in the missing bits and pieces is essential.