Some guesses off the top of my head:
1. Four hours is a long time to be sitting in a room with a number two pencil and a bubble sheet.
2. Many students don't read a lot any more. I teach some bright male students who can read well enough, as long the reading consists of cheats for video games and short articles in Maxim or GQ. Most of my female students read more than the males since they're e-mailing all the time. None of them, however, read newspapers. A few read novels. A lot of them, both male and female, read the Internet, which is often fragmented and presented in tiny bits.
3. A lot of the standards that we used to have back when I was child in the late Stone Age (circa 1965) have vanished. I know teachers who smile and brag that they can't spell. Grammar lessons are scattered in the curriculum; it used to be the main subject of learning. Proper diction, smooth prose, and elegant style are virtually ignored in the modern English class and rarely imposed on a student, unless it is part of a lesson about an author or a specific book.
4. Just observing my society over the years I've noticed a decline in clarity as more and more of us adopt legalese as a our "main mode of communication" (speaking). At the same time, comedy has devolved from wit and banter (Newhart, Tom Lehrer, the Marx Brothers) into crudity and grossness ("American Pie," Larry the Cable Guy). You have to understand something about language to catch the jokes in a Marx Brothers movie; shouting a vulgarism followed by a fart doesn't require much verbal understanding.
5. Compare the lyrics of Motown to the lyrics of a rap artists.
6. And let me tell you about the science teacher who once told me with some apparent pride that "I don't read books."
So those are some ideas. Overall, it doesn't surprise me that the reading scores of today's students would decline.
My two cents worth.
Allow me to add my $.02 to yours.
My nephew is 14 years old, and I have noticed that he and his friends have a palpable antipathy to learning.
I weep for the future.
Wait until they are sentenced to spend long workdays in a cubicle, a small office, or worst of all, a 90 minute department meeting where some moron higher on the chain speaks of the importance of how the team must come together, 'think outside the box,' and help craft a new 'pair-a-dig-um.'
No joke, at one of my old companies the head of the department was a complete moron who had just been around forever and pronounced 'paradigm' as 'pair-a-dig-um.'
I never missed that company.