various reasons:
1. i read the first book, and thought it was a rewrite of all the boarding school stories i'd read as a kid myself, just with the "magic" twist.
2. i got about three chapters into book 2, and lost the will to live - formulaic writing bores me to tears, unless it's particularly well done, and rowling doesn't.
3. the hype is enough to make me never read another one. i hate being told what to do, think or enjoy, and so rarely follow the crowd.
4. everyone witters on about it being popular because of the need for something supramundane (i.e., the magical content), but actually, there's no real magic as such, only technology.
5. potter himself is an annoying, pi little git, who needs a fucking good slap, and a tin of baked beans rubbed in his face.
6. partly, it's a kids' book, and i do find that a lot of them lack the depth and complexity i often require in a good read, but then pullman and lewis achieve both - i love both those series.
having said all of which, i know kids who were functionally illiterate for years, who suddenly learned to read so they could read the hp books, so i rate them for that reason. and so many people think they're great, so they must have something going for them. i'm just not motivated to put in the effort to find out.