...Which basically boils down to the idea that the reason they're as successful as they are is primarily because they are aimed at a teenaged audience, which means an audience likely too young to realise just how incredibly cliched and done-before the ideas within are.
Case in point, a conversation I had with someone about the books in question:
Other person: Hey, you know the Twilight books?
Me: Aren't those the ones that are just Kelley Armstrong and Lauren K. Hamilton books but with all the sex removed?
Other person: Who?