Why is To Kill a Mockingbird so difficult?

Kids don't read it because it's difficult--but why is it difficult?
To Kill a Mockingbird is the very text that William Broz used as his prime example in his article, "Not Reading: The 800-pound Mockingbird in the classroom."  Broz argues that most students who are assigned the book don't actually read it.  Broz doesn't mention the text's difficulty in his article, but I wonder if his students--and many others--aren't reading it not only for the obvious reasons (have other priorities; think they can get away with not reading; don't like reading, period; want to read other stuff; etc.), but also because the book is so difficult. I certainly find that the book is too hard for at least half of my ninth graders. But why?  The difficulty is not captured in the book's lexile score, which is about the same as The Fault in Our Stars, but students don't seem to have trouble reading the John Green book.  On the contrary, they gobble it up at astonishing rates.  What, then, makes Mockingbird so difficult?

It's not the vocabulary, sentence structure, or background knowledge
Mockingbird's difficulty is not because of its vocabulary, though that is not easy.  (Any book that uses "seldom", "assuaged"," apothecary", "Methodist", "strictures", "chattels", "piety", "stinginess", "Cornwall", "sustain", "impotent", "apparel", "dictum", "persecution", "Battle of Hastings", "read law" and "taciturn" in the first two pages is pretty difficult, but The Fault in Our Stars manages to use sophisticated vocabulary and still be accessible.)

It's not the sentence structure or the background knowledge that's required, although again, many young readers will be put off by a book whose second page contains the sentence "Mindful of John Wesley's strictures on the use of many words in buying and selling, Simon made a pile practicing medicine, but in this pursuit he was unhappy lest he be tempted into doing what he knew was not for the glory of God, as the putting on of gold and costly apparel."  

Instead, what makes Mockingbird so difficult is the jumps in time, continuity and logic. Nearly every paragraph requires the reader to make an inference or catch a subtle subtext--and to make these inferences not only so as to appreciate shadings of meaning, but just to follow the basic events of the story.

A sample chapter (seven types of ambiguity!) Chapter 25, which I read out loud to my classes last Friday, is a case in point.  The chapter has a series of very confusing time-shifts, and neglects to mention some key information.  I go through the confusing sequence below, but the specifics aren't so important--what matters is that there are at least seven separate times in the first half of the chapter in which the main plot information is unspoken and needs to be inferred.

1. The chapter starts in medias res, with Jem telling Scout to put "him" out on the back steps.  We don't know what or who is to be put out. A few lines later we learn that it's a "small creature," and that Scout does put him out, scooping him up, putting him on the bottom step, and going back to her cot.

2. Then, after a bit of scene-setting (it's September, they're still sleeping on the porch, etc.), we hear that a "roly-poly" is in the house.  It might occur to us to wonder if this is the creature Scout put out on the back steps, but we wonder, if so, what the thing is doing back inside.  

3. Putting down her book, Scout watches the roly-poly for a while, and then, feeling sleepy, she "decided to end things."  (Not all of my students realized that she was going to kill the bug.)  

4. Scout says, "My hand was going down on him when Jem spoke."  The next paragraph begins, "Jem was scowling.  It was part of the stage he was going through"...  Again we are thrown off balance: Jem "spoke," but what did he say?  We aren't told; we are supposed to remember the first line of the chapter, which was, "Don't do that, Scout. Set him out on the back steps."  But the narration goes on, not mentioning anything about Scout's scooping him up and putting him on the back steps, so we're not really sure.

5. Over the next few paragraphs Scout lies on her cot thinking about things, and soon, she says, she is "wide awake, remembering what Dill had told me."  What had Dill told her, we wonder; are we supposed to know?  

6. She goes on to describe a time Dill and Jem were walking back from swimming at the local swimming hole and get picked up in the car by Atticus and Calpurnia, who are headed to Tom Robinson's house.  Now, if we remember the previous chapter we may wonder if this is the time, the previous month, when Atticus and Calpurnia go out to tell Tom's wife that he has been shot, but we certainly are never told this straight out, and at least half of my students had no idea that this was the particular day that Scout is remembering.

7. Those students would have been confused, then, when Helen Robinson ("Who's Helen?" one of my students called out as I was reading) suddenly collapses just before she reaches Atticus.  Not only has Atticus not told Helen that her husband is dead, Harper Lee hasn't told us that Helen is collapsing because she sees the truth in Atticus's face--nor even that Atticus is there to deliver the news.

Conclusion: maybe we shouldn't assign it so widely?
To Kill a Mockingbird is a wonderful, intricately constructed novel, but it is often needlessly obscure, and I sympathize with those of my students who say they would never want to read it to themselves.  My 13-year old son was given the book a few years ago by his grandparents, and it has sat unread on his shelf to this day. In short, I think we should reconsider assigning it as widely as we do.