Good question (because if you know why startups fail, you could minimize - not prevent - the risks of failing for the same reasons).
The best answer (to most questions) is the one based on data (not opinions) as the famous/funny quote goes: "I believe in GOD, all others must bring data". Therefore 'My' answer is based on research done by CBInsights in which they reviewed 101 postmortems of startups that failed and then deduced the top 20 reasons. Here's the link:
https://www.cbinsights.com/research/startup-failure-reasons-top/
Here's the top 10 reasons:
1. No market need.
2. Ran out of cash.
3. Not the right team (I would put this as #2 - opinion based on working with over 100 startups).
4. Get outcompeted.
5. Pricing / cost issues.
6. User unfriendly product (I would move more down the list)
7. Product without a business model (I would move this to #3)
8. Poor Marketing.
9. Ignore Customers (I think this is very similar to #1)
10. Product Mistimed.
Happy to help on any other issue you might have
Best of luck