Why do fourteen year old girls who like twilight all the sudden are in love with vampires?
In my opinion, I think it’s an indication that popular trends are shifting. Girls used to fawn over the shining white knight or "Prince Charming" type character as the love of their lives. But welcome now to Edward Cullen who is basically the antithesis since he’s dark, brooding and definitely has some baggage.
Girls and women obsess over both these male personas since they both ultimately cherish their soul mates and lives only to protect them. The difference is now that the coveted perfect man is somewhat damaged and needs the love of a good woman to reform him. He also wants nothing but her love and would die for it.
Personally I don’t think it has so much to do with "vampires" as just the reformed bad boy character which some women have always found attractive.