1. Love story - Women of all ages love to read love stories with handsome men.
2. Bland lead - She is pretty darn bland yet this immortal falls in love with her and wants to make her immortal with him. That is an unbelievable gift, immortality. She is so bland every woman can project themselves into her.
3. Paranormal Romance - This genre is between romance and erotic lit at your bookstore. It's freaky romance stories with ghosts, werewolves and vampires. It is really just more of "women falls for men she can't have".
4. Abstinence Tale - While Christians will dislike the supernatural features, it teaches the young girls to wait to have sex and is constantly talking about not doing the deed (sex or turning vampire). Just substitute sex for vampie turning, and this book is a Victorian Era morality story.
5. Young Protagonist Discovering Herself - Many people want to go back to their 17 year old self and relive those days. This book allows that for so many of these women.
6. Amazing Male Love Interest - This guy is an immortal but falls in love with the bland new girl in school. Look at many adult America men. Over 70% are overweight and 1/3 are obese (source). These women want to dream that the guy they are stuck with was a fit, immortal, beautiful, song & poetry writing, forever young man.
A Venn Diagram would have several circles: wholesome virgins, paranormal romance readers, teens, romantics, middle aged women married to schleps, overweight females, people who will read it to mock others. That is a big mess of circles.