Why do certain books catch on and others, apparently better, fizzle? Marketing has a lot to do with it, so does timing, the cover, a review by somebody like Oprah, but still nobody can predict what will work and what won't. As William Goldman said about trying to predict movie hits, ultimately nobody knows anything.
A case in point: the Fifty Shades trilogy, erotic novels that have just passed the ten million sales mark after only six weeks. Publishers Weekly reports that Anthony Chrico, president of Knopf Doubleday, said the division has been reprinting the books "on a weekly basis since publication" with, he added, certain daily reprints of over 950,000 copies.
Confession: I haven't read any of the trilogy, so I'm not going to judge them, only to note that a lot of people have said there have been many better books in that genre that haven't caught on, and that on the first page of "Fifty Shades of Grey" the author, E. L. James uses that most basic trick of first-person novels, having the narrator look at herself in the mirror and tell us what she sees. It's an easy way of leting the reader know what the narrator looks like, and most writing teachers would advise you not to use it because it's so obvious a device.
I don't think it pays to be snooty about books that sell well, obviously they're hitting some kind of nerve. But what it is that causes that effect unfortunately remains a mystery (if you know, let me in on it!). All you can do is write the best book you can, get it out there, market it as much as you can, and the rest is with the gods (or, in this case, the readers).