You know what, I don't really have a reason. Probably the same reason most books are so appealing: entertainment and escapism. It's funny, because I was always one of those people who swore they'd never get into the whole "urban fantasy" thing, and now they are some of my favorite books. I mean, anything I read is going to have to meet the basic criteria -- well written, believeable characters, well-paced plot etc. But for some reason, I find the books you listed (vampires, romances and mysteries, oh my) to be the most compelling.
I used to be the same way. Every time I tried to read a vampire novel, I couldn't get past the fact that it was totally impossible. The first time I tried to read the Anita Blake books by Laurell K Hamilton, I got to like the 4th page of the first book. I mean, the protagonist was covered in chicken blood and raising a zombie. Hello.
Honestly, after 9/11 I became totally obsessed with cable news and reading every newspaper I could get my hands on. It was around that time I started reading more fantasitcal novels. Maybe I had too much real world in real life. I don't know...
I like dark, Gothic things and many of the vampire novels fit this theme. I don't have any other reason, but i don't just like vamps. I like were-creatures, witches and fairies and all kinds of other supernatural creatures.
I cannot get into the vampire novels. Don't watch the TV series about them either. I just started to enjoy 'other world' novels. Not sure of the exact label, but they are fantasy novels. I guess I am just rooted in every day life and every day mysteries.
My greatest hurdle to get over when I first read it was the structure of the story. Epistolary fiction is sometimes hard to immerse myself in, though Stoker did a yeoman's job differentiating the voices of each character enough to work the suspension of disbelief.
I found Dracula just went on. And on. And on. And on. I think it was quite 'street' in its day but the downside to that is it hasn't aged well at all. The only vampire book I ever warmed to was Stephen King's Christine. And I don't even like Stephen King!
Because they're bored with historical romances? Because vampires are finally known as cute and can be diametrically opposite evil? Because angst is now in? Because black is romantic? Because mysteries...? Well, I dunno. People have always liked mysteries, unless of course there are vampires in them. But basically, almost all vampire related books could be termed a mystery.