A lot of factors are important.
Your product must suit the market, your product should actually solve a problem and not add another problem. It should be catered to the target audience too but unless it makes a solution for them, it will not sell. Costing is also another factor.
Pinpointing one factor to be most important in business is punitive in my opinion.
This is a main key point when discussing investment elements. In general, all the details are important especially when to start. You can't deny any factor. The example of corona can explain it, when many businesses in touristic sector ignored all the warnings about the pandemic which later forces a global lockup. Assumptions about loss says that billions of dollars lost because they didn't take the right decision in the exact time.