I think people liked it because the early books are kinda evocative of something really whimsical and Dahl-esque. The house system, the castle, the owls, etc. it has a very flushed out initial setting that you (especially as a little kid with limited capacity to critically analyze it) can imagine fitting into and that adds a lot of character to what's happening.

It struggles as it goes on because it becomes more and more clear that JK is not capable of critical analysis either and that all the evocative stuff is going to basically amount to nothing. She never really builds on top of any of the foundations she sets down, she would rather bring up a new evocative thing instead. Quidditch is a great example: it sucks, the rules are insane, you cannot understand the stakes and it's impossible to care about, but she never really does anything with the funny premise that the wizards are all bent out of shape over something so silly.

A huge issue with this is that Rowling is, at her core, an uncritical Br*tish person who is not capable of recognizing the problems with the tropes she trafficked in or the very obvious subtext to the world she created: the wizards ARE a supremacist closed society that subjugates every sapient magical being to wizards, that have no regard for the well-being of other humans, and are committed to defending a clearly dysfunctional status quo that Rowling cannot countenance any criticism of. So as the books go on they become less whimsical and more horrific without her noticing. The book series literally ends with the protagonist wondering whether the chattel slave he inherited will bring him food, and you are not supposed to think that says anything negative about Harry.

I think she also succumbed to movie brain, a lot of stuff aligns more with the movie depictions of things over time, and her writing also starts to seem like she's writing with an eye to how a movie would look. Given her weakness as a writer pivoting into a different craft she is even worse at was a bad idea.

Edit: the TL;DR is that the strength of the series' world building is almost entirely evocative imagery and tropes that are easy to imagine fitting in with. Which is to say you can imagine buying and filling your room with Harry Potter objects because you have a clear idea what that would mean.