The answer to this is probably the same boring answer that so often explains great successes: it wasn't something structurally novel about the Harry Potter books, but simply that Rowling did a great job writing them.
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Also: Randomness. There must be hundreds of novels better than Harry Potter series that never made it big.
As someone who is constantly scouring bookshelves for good books for 7-10 year olds, I can say this is false, at least as regards published novels.
