Harry Potter was an easily digestible, fun, and not too challenging read for kids and young adults, many of whom weren't reading anything at all prior to that series. The setting and simplistic story also makes it easily accessible and familiar enough while it also features a bit of fantasy elements to keep it feeling new and fresh, but not too much fantasy so as to push away people that aren't familiar with that kind of stuff. It had a a combination of elements that made it perfectly poised to take off the way it did.

I'm not trying to say that they are great books, just that it is incredibly easy to understand why they got as big as they did even though there are tons of other options that do every aspect better.