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.
Absolutely right, that's why many business trainers suggest starting with the question "what problem will our product solve?" and based on that, you can already understand what its main target audience will be.
And in my opinion, it is best when the business organizer is at the age when he matches the target audience, because then he sees it through the eyes of his clients and begins to clearly see the nuances that can help him in the future in building and developing the business.