I'm going to go out on a limb and say it was the wrong book at the right time. I remember when it first started getting popular there were a lot of comparisons made to Harry Potter, which didn't make sense to me because the books had nothing in common. But the more I thought about it, the more I started to think of Twilight as less of a rival to Harry Potter and more as a successor. Think about it, the harry potter books created a massive generation of young readers (arguably more girls than boys). Now this generation has reached the tween/teen ages and they are looking for another series to become obsessed with. Enter twilight. It's non-threatening, romantic (a matter of opinion), and has a descent amount of lore to chew on. It came at the right time and the Harry Potter generation ate it up with a spoon.
Be prepared to see this happen again, as they outgrow twilight and start looking for adult novels. Though I originally disliked twilight fiercely, if it means more people are reading, then at least its doing something right. As for the boys that liked Harry Potter, they seemed to gravitate towards Percy Jackson and the Olympians, which is a much greater series than Twilight, in my opinion. But then again, I'm a guy.