The reason books sell has little to do with content quality. At least initially. Just like any other product on the retail market, books sell because readers know about them.
Some books have the advantage of author platform, or industry applicability, and can hit strong out of the gate. The more nebulous quality of fiction can result in a slower start, but a much longer shelf-life.
The point is this, know your market and leverage your resources to meet those potential readers. Outline a marketing plan with your self-publisher that takes advantage of internet marketing and retail resources like Amazon who has pre-qualified readers based on profiles and virtual book pairings.
Work smarter. Work harder. Become a successful self-published author.