So many reasons. Aside from the quality of the content, most issues come down to marketing. Do you know the book on marketing a book? https://www.amazon.com/1001-Ways-Market-Books-World-ebook/dp/B01LXAK729/ref=sr_1_1?s=books&ie=UTF8&qid=1490965601&sr=1-1&keywords=1001+ways+to+market+your+books