my theory:
1) school-era nostalgia, only now the nerds can be athletes and the wallflowers popular due to that most intangible of fancies: MAGICK! anyone in the potterverse can be a great wizard and thus stave off the cruelty of high school torment, and perhaps even score with the hotties and get invited to cool adventures/parties! even though harry's a moody fuck, we are always aware that his life ain't so bad because OMG HE'S A FUCKING WIZARD. try being harry without the grand adventures, loyal friends, and magical superpowers -- now THAT sucks!
2) simple, easily digested plots with well-developed casts that lend themselves particularly well to escapism and self-insertion. the school setting is particularly well made for self-insertion; hence the obscene number of fanfics (and complete novel rewrites)!
3) magickal world is anodyne against the cruel mundanity of the real world; many of the folks "discovering" harry potter -- especially the adults -- have never read fantasy, and the realization that they can imagine the fantastic is like an immediate amelioration of some component of their hatefully dull, unsuccessful little lives. i can't count the number of fans who associate this feeling of "imagination" exclusively with harry potter, and they often won't try other books because they feel that by doing so they tarnish the pure recollection of first reading the hp books.
4) some well-read folks read and enjoy harry potter because the style likewise evokes a sense of nostalgia for the roald dahl and similar books of their childhood, and because as far as escapism goes: hey, at least it isn't piers anthony!