If you want another sad-making observation, the teachers say teens are increasingly finding late-20th-century kids' books hard to read, because the language is too complex for them to follow—they're used to first-person with very simple sentences with redundancy for anything important (so it's harder to miss), and publishers are chasing simplicity/clarity-at-all-costs hard for fear of alienating any of their shrinking reader-base, which of course re-enforces that decline in ability to handle sentence- and paragraph-level complexity—even goes for "advanced" readers, I'm told. This part's been heading that way for a while, but I guess has gotten much worse fast over the last 5ish years.
Basically, the genres that people still read (YA and romance) are trending hard toward first-person and very simple language. If you write in third publishers and agents will either tell you to switch outright, or you'll get a lot of "seems too distant and impersonal" and such, which is just them asking for first person without saying it. Some get through anyway, especially from authors who got their start 10+ years ago and have more sway, but first is strongly preferred.
That doesn't surprise me at all, I'm sure 20th century teachers found the same trend in 20th century students trying to read 19th century books. It's a shame, but it is what it is. I wouldn't draw too many conclusions from this, except that we should do what we can to encourage kids to read the classics despite (or even because) of the difficulty.
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.
Or take Shakespeare for example; he wrote for the unwashed masses yet his writings are considered sophisticated and challenging today. I have an anthology of English renaissance drama from various playwrights that I revisit from time to time. The stories are usually quite crude and funny (example: https://en.wikipedia.org/wiki/%27Tis_Pity_She%27s_a_Whore), but understanding them well enough to appreciate that humor can be a chore. I don't think these stories were considered difficult originally, but became so over time.