If you did a PCA on the reasons reading has gone down, this would get a very low score.

I can take most young people today, give them access to all the paperbacks in the world (and despite the point of the article, it is very easy to find paperbacks!) and their reading will barely change.

The culture has shifted. Screens are a likely problem. Books - particularly paperbacks - were handy because they could be used to in times of boredom. Waiting at the doctor's office. Commuting to work/school (if you're not driving). In the evening when nothing good was on TV and your computer is turned off. When you didn't want to tie up the phone line talking to friends.

Now there are many more alternatives to fill those spaces.