Harry Potter has some issues, but for children's fiction it's better than a lot of series.

What do you mean by strange and mean?

An absurd amount of marketing, mainly. Very easy to shove YA/childrens books down kids throats, they don't have a lot of natural exposure to literature. Fuck, eragorn was the best example of the YA industry pushing a bad series (they even tried a movie series), I remember loaning it from my school library and being legitimately confused as to why it was becoming popular. I ended up finding a weird romance+fantasy series at the time that I largely consider as not being actually good, but remember finding it way more engaging. Maybe it's better now that kids are largely terminally online.

It's really my biggest gripe with it. There's better fantasy, better wizard centric fantasy, and better YA books out there. It's not great by any means, and I'm not surprised that I dropped the series without finishing it as a kid because I was reading much better stuff by the time the last few books came out.

I'd guess it's because it captured the demand for english-language isekai at a mid reading level, and snowballed with hype around new releases, which quickly got rolled into the movie franchise as well. For 15 years there was a new release almost every year, between the books and the movies, so you couldn't really avoid hearing buzz about it. If it was just one book without regular injections of hype into the public consciousness, it'd probably be largely forgotten.

Kids don't care so much about prose and they're usually too naive to pick up on political subtexts, at least consciously. As a kid I liked them for the escapist fantasy and the simple narrative.

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.