Because reading is hard to get into initially. That's why it's so important to give children a good, decent education, to train them to read, because once they get old, they become extremely reluctant to.
It's a habit you have to develop yourself. Not only that, I feel that quite a bit of literature is outdated and unpopular. I read a hell of a lot of books, mostly non-fiction books dealing with science, history and politics. Not only does the vast majority of 20-something year olds find these topics boring, but the fiction books they should be reading aren't much better.
My brother's a pretty smart guy, but he doesn't read many books anymore. However, I was able to get him to read Catch-22, and he enjoyed it.
It's really not a person's fault that they don't like to read. They just haven't been introduced to good books. And when I say "good" books, I mean genuinely entertaining books like Catch 22 or Brave New World or Catcher in the Rye, and not boring, stodgy old books like "pride and prejudice", which has to be one of THE most asinine, outdated, and just plain boring books I've ever read.
And of course, there are people like Kanye West who recently said that he was a "proud non-book reader". But in all honesty it's the fact that books have lost the ability to sell themselves. Those who do not read books, never learn how great they can be.
To anyone here who wants to read, but isn't sure what book to start with, I would always recommend "Catch 22" or the HHGTTG.