I've always been a huge reader. Picked up Green Eggs & Ham and never looked back. Started on Harry Potter when I was... 6? Maybe younger? I dunno. Redwall at 7. Artemis Fowl & Series of Unfortunate Events at 8. I still love a good 'kids' book. Anyways, it sort of snowballed. It's a rare day that I don't have a book somewhere on my person.
It's a little hard for me to say why some kids don't hook on reading, just because my love of reading had absolutely nothing to do with my school environment. I was always reading way ahead of the rest of my class, and I was never bullied - I just read. When I hit Grade 7, though, we had a wicked sharp class, so we read Brave New World and 1984. The board dedicated to discussing themes had SEX in big bold letters, dead centre. Boy, that class rocked. Later English stuff was boring, except the Shakespeare, I've always been able to read it pretty easily. Richard III ftw.
I think there's something to the idea that kids who haven't learned to enjoy the benefits of reading, who are taught the skills and have to work at it, will see reading as busy-work, as is pretty much all schoolwork, rather than something that can have real rewards.
Strangely enough, my brother, who's very much like me in most ways, is not a big reader - he rejects book suggestions, and takes forever to get through a page - although with my constant nagging I've gotten him into some stuff (he's partway through Feast for Crows at the moment).
One thing I think it might be is that there a great many more accessible forms of storytelling out there - Television, Film, Video Games - They all serve to fill essentially the same role for me, and as such, as I've watched more and more TV Shows and whatnot, my reading level has gone down, it's more sporadic (which is probably an improvement from non-stop). They're different mediums with different strengths and styles, but they fulfill the same role, and I imagine for a kid, having to struggle slowly through a book isn't worth a story if you can just get it from TV, which they're very familiar with. Maybe that's why I read so much when I was young - I didn't get cable TV 'til I was 9.