I think it would be interesting to ask, “Why Rowling?” because I hear that asked, esp. by writer friends who are frustrated by her success. It’s not their own work they think is better, but they invariably mention Philip Pullman as much better (I agree at least regarding the first novel in His Dark Materials) and other fantasy writers they admire as more original.
I think it’s accessibility. Rowling is a very non-intimidating writer. Much fantasy has a kind of elevated style and a the characters are remote and hard to identify with. Rowling integrates the fantasy with realism kids can identify with – Harry struggles with being misunderstood by his peers, at times, and worries about which girls like him, hates school, loves sports, and despises his step family. He’s a normal kid.
But it’s not just about characters, its also about the style. Rowling is not on a higher plane than her readers. It’s considered bad form to explain too much in fantasy, since intelligent readers will figure it out, but Rowling set herself well up to explain everything since Harry was new to the fantasy world himself, and that makes it easier for kids.
There is definitely hype/snowballe effect in play, but in sum, Rowling is much easier for the non-fantasy reader to enjoy. She isn’t writing for fantasy fans, just for kids. Indeed, she doesn’t read fantasy herself much (she must have read some), and it so she’s an outsider, connecting more with non-fans of the genre and bugging the shit out of many people who follow it closely.
Ironically, I’m in the middle of my own MG fantasy novel (writing, I mean), which sort of happened by accident (it’s fantasy in the broader sense of meaning “not conforming to the rules of the natural world as we know them,” and not in the more limited sense where one encounters dragons and hippogriffs). When you set out to write a novel, you learn to appreciate that its hard work even to write a bad novel, and bad genre fiction is more demanding than bad literary fiction, since you have to know and conform to rules of the genre and basic rules of fiction writing.