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.
Self Publishing Advisor 
I agree with your thoughts as I myself have found several excellent and well-written books that are not popular. promoting their books are not that extensive and hence even if they would create very high quality content, no possible reader knows about it. To promote a book is possible and effective with the advent of the Internet as the reach is far more extensive and more and more possibilities and means can be implemented.