Harry Potter could just as well have been likened to the earlier Hobbit, Lord of the Rings or Watership Down phenomena as being due to the kids in the school. But there are plenty of adult-level classic literature novels about children of various ages. I know it’s long been a genre but it really has exploded from a bit before Harry, I think, and I still hold that strong readers never really go for them—’cept maybe precocious 5th graders reading about naughty high schoolers!
LOTR and Watership Down didn’t make their writers royalty rich. They were successful books, and over the long haul might be more successful than Potter, but over the short term, which is really what publishers are thinking about, they’ve got nothing on Potter, or it’s “predecessor” Goosebumps, or “successor” Twilight. These books series are licenses to print money.
It’s not just a matter of the books being “about children”, you can make a protagonist any age you like, it’s a matter of the books being about children that other children can understand. Children facing children problems. Trying to find their place in the world, dealing with being “different”, not feeling their parents understand them or are fair. It’s not the strength of the reader or the language, it’s the CONTENT being within the kids world view. LOTR is a brilliant book, and has great lessons for kids, but there really aren’t kid issues. Same with Watership Down. I read all those when I was tween age, but there was no identifying with them, nobody in those books was me. And that’s what’s driving the current push, authors have finally figured out how to put in characters that kid/ YA reader can not just like but UNDERSTAND.