As someone who is addicted to reading (literally!), I have a few theories about why people don't read.
1. They grew up in homes where the parents didn't encourage reading. The parents didn't read, and didn't read to the kids. In my case, both parents read to me, I saw them reading, and most importantly, I had an adult (grandmother) who not only loved reading but would talk to me about books and would recommend books and stories to me. Having someone you look up to say "You know, I was reading this book and thought you would like it" makes a real difference.
2. Schools too often take the "fun" out of reading by making is compulsory. Let a kid choose the book, and you're much more likely said kid will read it. Even if the book is too advanced for the kid, let them figure it out on their own. I started reading Jane Eyre when I was 10, and at first could only get through the school days part of the book. As I got older, I advanced through Rochester Hall, and finally was in my late 20's before I was able to read the book cover to cover. But even before then, I read and re-read the parts I could comprehend and enjoy.
3. Censor their reading sensibly. In my case, I babysat for a couple that had some very ...interesting... books, which resulted in my asking my mother what a specific term meant (I was 12). She swallowed hard, but explained. Since she knew she couldn't truly censor my reading, instead she asked that I come to her with any questions and we'd discuss them. I learned a lot that way.
4. Give kids time to read. Too often parents fill the kids days with activity after activity, they don't get the time to just sit and play, or read.
5. Let kids read to you, instead of you always reading to them. Even if said kid can't read all the words yet, letting them tell you the story (based on the pictures) is a great way for them to exercise their imaginations.
Finally, 6. Give books as gifts. All my nieces and nephews have gotten books from me from the beginning. My one rule is that I never give a book to them that I haven't previously read (at least a copy of). And I always include a note with the book explaining why I chose said book -- oftentimes, because I'd read it when I was about their age and had loved it.
