Duncan Watts has a fascinating, brief piece in the NYT Magazine today about the "rich get richer" effect and how cultural mega-hits don't have much to do with many individuals independently liking a given song or book. Rather, the success of a successful book or song and the failure of an extremely similar competitor has mostly to do with conditions fertile to sparking a "collective liking" effect (people buying a book since everyone else is buying the book, compounded over and over). Cultivating these conditions is something every author (or musician) thinks about. While you do your best to foster the butterfly effect and try to capitalize on random positive fluctuations in the hopes that maybe, just maybe, it can lead to your book "blowing up" -- and entrepreneurs think the same way when trying to propagate a new idea or spur potential customers to action -- the reality seems to be that this is truly a chaotic process.
People love to ascribe specific reasons for why one product succeeded over a very similar competitor. Sometimes these reasons are accurate, but sometimes I think a humble shrug can be the most honest explanation.