Generally the answer is lack of action.

I remember having a talk with a person at an Internet Marketing Party in Austin (everyone should travel to Austin + attend a few of these) about her project.

She had determined she required $80K to launch.

She had wasted... er, invested... 2x+ years asking investors for money.

Over a 15 minute period, I mapped out how I'd use https://Meetup.com for $100/year to bootstrap a business like hers.

She did.

She was making 5x figures a month in a few months.

This was her first business + she had no previous experience.

Moral: Action trumps all else.

Tip: If you can't figure out how to turn an idea into 5x figures a month in a few months... Either hire someone to help you design your monetization or come up with another idea.