It's popular initially, I think, because it has a really strong start and an interesting main character. However, like the above poster mentioned, it swiftly dissolves into terrible sueness- for example (these aren't terrible spoilers, but they are spoilers nonetheless), the protagonist's main love interest ends up having initially fled from her and being so offputting because she's, basically, his lifemate or whatever the popular term is nowadays. And he angsts about drinking blood and being a vampire and stuff. What else.... Oh, well, aside from the general cliches, etc., there's some really creepy stuff like the main character ignoring and losing total 100% touch with all her friends once she gets into a relationship.
So.... yeah. It starts out really impressively, and, sadly, Sues sell in the 'vampire' genre.
OH YEAH and I forgot- at the end, it seems like the writer just gave up trying NOT to be Sueish and... well, I can't ruin it for you, since you mentioned you have interest in the book, but. Well. It made me cringe.