Here's my take on this question:
A startup's life can be separated into 3 stages, each having its own primary reason for failure:
1. Idea stage (from conception to go-to-market) - the main reason for failure at this stage is the team breakup (which can have many reasons in its turn)
2. Product stage (from go-to-market to product-market-fit) - the main reason for failure, simply put, is when you discover that you've built something no one wants (or at least not enough people want bad enough to pay for it or even use it for free)
3. Growth stage - the most common reason for failure is premature scaling, building the company too large to sustain with revenues, i.e. the growth is slower than expected and eventually you run out of money and investors don't want to invest more.