I think what's so shocking is these books are already so damn simple—they weren't considered challenging to their target age group even 20 years ago—but they're still too complex for 'em. It's like if there are two clauses they can't keep the first one in their head long enough to make sense of the whole thing, and god forbid you need to track an idea across multiple sentences.

I have a suspicion that the trend dates back much earlier (60s? 70s?), and owes to de-emphasizing poetry in English classes, since reading poetry requires heavy tracking of complex context over long stretches of text, making even fairly-complex prose mostly seem easy by comparison. This may just be a continuation of that trend—kids' lit gradually getting simpler in response to declining literacy that is itself due to shifts in curriculum focus, causing an ongoing, further decline.