Well, there is no real "why." All you can say is the public liked the premise, the style, the reading experience, something — and people started telling one another about this book, and voila, you have a success. Then come the detractors screaming that the new success is a travesty, "badly written," "a rip off," and they're offended. Meaningless. The readers have made up their minds, and they're talking about the book. And so we go on.
All this mystery about success is a good thing. No one can control what readers will love and elevate. No one can "make" a bestseller. And that gives everybody a chance.