Hands down it's marketing, with a side of product-market fit. The latter was part of why Twilight and then Fifty Shades of Grey exploded. Product-market fit for

Hands down it's marketing, with a side of product-market fit. The latter was part of why Twilight and then Fifty Shades of Grey exploded. Product-market fit for a book relies more on engagingness, and pressing the right buttons of a certain demographic, than it does on literary or academic measures of quality (which are subjective, of course).