Technology plays a big part, as has been said. It's a lot easier and faster to plop on the sofa and watch harry potter. For some people, even the smallest Harry Potter book could take a week.
Another factor is (as with everything else) parenting. If the parents don't motivate or encourage their child to read, chances are that the kid isn't going to do it themselves, and with kids, it's all about motivation. Tell a child reading is cool and they're going to start reading. Then they find the psychological and intellectual benefits and continue to read. It's a chain process, but if it isn't started, it's very hard to build.