The quick answer - control and speed.
As a debut novelist, I want to control what happens and I am not sure that a traditional publisher would be prepared to take on the work of a new author like me in the current climate. Hope I haven't got this wrong, but it's just being realistic. There are a lot of new authors out there in an already crowded market.
I suppose many new authors lack confidence (or at least occasionally racked with self-doubt), and this seems the natural state for a writer - but it is also partly impatience. For myself, the ambition to publish a novel has burned brightly for many years. Having closed my business earlier this year, I now have the time to devote to my writing (at last). Full-time work and caring responsibilities are not compatible with completing and publishing a novel, so now I can stay at home to help my wife look after our son and still have time to work on a book.
Impatience means I want to get on with publishing the book. Yes, there are risks - quality could suffer, I accept that. A large, traditional publishing house would have copy editors, sub-editors and proof readers making sure that everything is perfect. For my book, there's just me - author, editor, publisher and marketing manager, with my wife (thankfully) doing the proof reading.
Why self publish? For my first book, at the moment, I don't think I have any choice. Maybe that will change in the future if the book gets noticed. If not, I'll publish my second book and try again.