I can see why it's a popular design; they're very striking and strongly designed to appeal to teenage girls for the following reasons:
- the use of teenage models
- the luxurious dresses invoking a romantic feel
- the melodramatic atmosphere
- the element of mystery when we have a girl wearing a ball dress in an unlikely location (often in the woods). It makes the viewer want to know why she's wearing that dress and where she's going, which makes it more likely they'll buy the book!