One reason might be the ease of self-publishing with many considering their life story and imagined fantasy of value to a wider public, and another being the generally poor state of public education in which literacy is no longer considered the minimal requirement for high school graduation.
The encouragement of reading by parents may also be less prevalent with some parents failing to read first to and then with their toddlers, this being a time when the child can naturally learn reading since the mind has an inherent capacity to induct the nature of reading just as it does the grammatical structure of the language of the nation into which they are born.
A smaller factor, I believe, is that reading involves a loss of self-control while immersed in the written words which is unconsciously scary for anxious youth. I having known good readers who never read, preferring video game play which is heavily obsessive-compulsive, one of the mind's natural ways to reduce anxiety and thus powerfully motivating.