The short answer is this: I’ll be darned if I know!
The long answer, however, is this: Many, many factors.
First, you have to have a good story, and/or a book with wide appeal. Second, it must be well-designed. Third, you have to get it to market.
A good story has a beginning, a middle, and an end. Something to hook the reader and let him in, hoping to discover more and wanting to know more.
A book with wide appeal is something to which many people can relate. In fiction, that can be all over the map, so to speak; in non-fiction, it could be as simple as cooking, or self-help, or entertainment, or information on a common illness or disorder.
That simply means that it must have universal appeal—a book about your family will be interesting to your family, but not necessarily anyone else (unless you’re famous). A book about something you overcame or your own life story may be intriguing to some, but not to many (unless you’re famous).
And of course, you can’t price it too high.
Timing helps too—you don’t want to release a holiday book in the spring, for example.
Then you have marketing. This means exposure. If you don’t have a website or do nothing with it, if you don’t have a blog or never post new material, if you only market to other writers, etc., etc., you will not sell books. If you never do events or festivals or appearances or speaking engagements, you will not sell books.
If you do all these things, consistently, you will sell books. Probably.
The single biggest factor for marketability is luck.
Just stop and let that sink in for a moment.
You’ve all seen really crappy books, or even just a short story sold as a “book,” that climbs the Amazon charts. Ugh. And good ones, from unknown authors, that just sell and sell and sell.
WHAT THE HECK?
Luck. Pure and simple. Like winning the lottery. I can’t explain it and neither can you—if you have the awesomest story ever, and the most brilliant illustrations, but sometimes that just can’t compare to this type of luck.
What does this mean? Not a thing. It certainly doesn’t mean you can’t or shouldn’t write or draw or try to sell books—that won’t get you a darned thing, and that’s for sure. Keep trying, keep working on it—why?
Because that’s who you are.