Usually some well-funded competitor comes up later with the same idea and takes all the market.
My advice: Don't bother starting a side project unless it's really niche, easy to implement and you just want to make enough money to replace your own income (the market has to be small).
Unless you personally know people who have tons of money to invest in your side project, then it's a complete waste of time. It has to be people first.
9 out of 10 businesses fail and it has nothing to do with abilities, drive or persistence; it's only about luck.
I've met a 17 year old who built a wordpress website that generated $10K per month after just 6 months (though it already had tons of traffic in the first month) and couldn't write a single line of code. I've heard of hundreds of similar stories.
Some random person somewhere on planet earth randomly puts together an app because they think it's cool without any thought whatsoever; it blows up in popularity out of nowhere; then some smart engineers/people who actually understand the potential find out about it early enough and join up in exchange for a stake in the project; that's how actual smart people do business.