I dunno, I'm sure there's a more complicated and interconnected series of events which lead to them truly being popular, not least of which was the movies, but in terms of how they're structured, it kind of makes sense to me why they were a successful fiction. The various different houses, even though they're mostly indistinguishable from one another internal to the books, give kids something to identify with and self-categorize into, which is something that teenagers kind of love doing in a struggle for identity. They're also part of the hidden world subgenre, which means it's even easier for tweens to self-insert into.

Then, I think it also helps that they're kind of poorly written, weirdly enough. Every character isn't usually a real, fleshed out individual, they're just an archetype, and a shorthand, a common trope. I think this is probably desirable for a tween audience, and I think probably also a simple to follow plot and set of plot elements is also more desirable. There's no lore to keep up with, it's just like you've taken a bunch of other tropes from other, better works and compressed them into an easily digestible series of books full of melodrama. It's not super hard to understand. Those other books, they're like the various PDAs and shit you'd see floating around in the 90's, they're explicit works of art constructed for a singular purpose. Harry potter is like an ipod touch, or an iphone, or something, it's just engineered to have more mass appeal at the expense of complexity and possibly quality.