I read that the theory is called "priming" and it goes like this:
The raw neurological capacity for critical thinking skills don't develop in the human brain until 20 years of age (and don't really complete development until around 25). So, in an effort at making that happen sooner and better, we prevail upon young teenagers to read famous "classics" that were written for adults (books like The Grapes of Wrath or The Great Gatsby or etc.). Further, the teacher leads class discussions about the book by asking probing questions and setting the historical context and giving writing assignments about the story's events. This is supposed to stimulate early development of the neural connections associated with critical thinking.
And it doesn't work. AT ALL. It just leaves the kids insanely bored. They can understand the overt plot (which is boring) and the historical setting when told about it (but they can't relate to it because it is nothing like what they have experienced so far in their own lives), but the real substance of what these works contain is just lost on them. Worse yet, it makes them DISLIKE reading! These negative associations motivate them to look for other forms of entertainment so they never really exercise those skills.
If we instead gave them reading material that was more relatable AND more entertaining by virtue of being written specifically FOR their age group, they will actually enjoy reading it. Even if they learn nothing else from it, the sheer fact that they are engaged while reading it means that they will be pushing the boundaries of their attention span, improving their vocabulary, exercise reading comprehension skills, and etc. And best of all many of them will seek more books to read for pleasure after having a positive experience like that, so they will be master readers by the time they hit college and are actually neurologically ready to read stories about the great depression or class struggles, and actually understand their significance.
This is just one element contributing to the problem. Others have pointed out many other elements that also contribute.