Did you consider the audience?

How many kids transitioning to young adults have enough grasp of historical literature or other media to say "what bollocks! This is super derivative! Let's do some high Arthurian fantasy instead!"? It got popular because it is approachable, entertaining, and knew it's audience.

Now, jkr might have picked this kind of audience because she new as an aspiring author the quality wasn't going to be tip top, but that's a different discussion altogether.