Lack of Capitol. Time, money, and energy. Some people have one or two, but not all three, and they fail because they simply can not sustain the growth phase of their business. Lack of Courage or Commitment. Building a business is always risky.
Some people perceive the risk as huge, others see it as a fun hobby, but there is risk.
Limit your potential losses. Understand the risks involved and enjoy the process. You will make mistakes. Learn from them and go on. Refusal to select and target an audience.