I blame Angst Goddess.
Let me start off saying that I don't get into AH fics. I've read only a few, and only liked a few of the few I've read. But I think Volturi Girl has a good point. I'd like to take it a step further.
The thing that grips us about Edward and Bella's love story is what the authors want to play with. It's the "Twilight Recipe for Salvation by Love". The lost man saved by the innocent, seemingly helpless woman, is an epic tale. Bella always ends up being stronger than Edward in a way, but it is Edward's strength in the end that saves them both. That is a powerful combination for a love story. Add unrealistically tortured souls haunted by tragic events, and you have a fanfic masterpiece.
As Gisellelx, Elise Shaw and I (we canon writers like to stick together) lament, it's also insanely easier to write a human fic rather than keep to the vamp world. I know because I tried a one-shot just to see how it went. It was fun, easy, and a hoot to do. It also taught me that I could, if I REALLY worked at it, write my own original story.
By the way, in my opinion, the Fanfic Recipe only works well with the names. I've said several times, if you take your favorite fic and replace the names with say, Fred and Wilma or Ricky and Lucy, they fall apart. Not only do they fall apart, they are hilarious to read. Like Romeo and Juliet, the remake only works if you have the plot to back up the epic love.