There are many reasons why authors self-publish their work and they are as diverse as the subject matter, here are my reasons.
Management- I don't think my work would fall into any specific genre and therefore getting a publisher interested is hard at the best and in most cases impossible as they are naturally only interested in what they think will sell, and my writing isn't main stream reading, unless you like H.P. Lovecraft and Gothic ghost stories.
Money- I cannot afford to pay for publicity when all I see for a book is $0.30c
Freedom of choice- I prefer to SP because I can choose the covers for my books and not get forced to accept a limited variety, many of which bear no relevance to my work. It is true that some of my friend's cover suggestions have given rise to stories but that is because we worked together for a year and she knew what type of cover fitted my story.
Who to trust- With a growing number of people 'claiming' to be publishing firms and turning out to be conmen, it is difficult to know who to trust.
I know there are many good cover designers out there but when the cheapest cover is about $50 that is a lot of sales to make to get my money back. A good cover can advance sales rapidly but as an Indie writer working on a shoestring budget, outlay Vs income is a priority for me.