Firstly, demand is not always a given. Sometimes it's built in to a market, other times it needs to be created. Other times, there's demand but no known solution if your work is novel, so you need to educate the market as to the benefits of your solution and to win trust in it.

Also, it's entirely possible to supply a product which meets a demand at good unit economics but fail because of other issues - recession, international political issues, governmental pressure, societal or social issues...

Finally, you need a market to be willing to accept a solution. It's entirely possible to have a better product, but lose the Comms/trust game in a winner takes all market situation.

Failure can come through issues in any of about nine different areas, but it always boils down to an issue in profitability or marketing. How that's expressed can take lots of forms. Hence you can simplify that far, and no further.