Her prose isn't great, but the world is. Somehow her writing creates a vivid world that people want to enter - and stay in.
You can be an extremely accomplished writer, getting everything right (plot points, structure, pacing, characterisation, actual technical writing) and yet all you end up with is words on a page.
When teaching myself how to write I spent a lot of time reading 'bad' fiction as well as 'good' and trying to get a feel for what made the difference. I learnt an unexpected amount from reading generic romance - which has a lot of really bad writing and a few excellent writers; as well as reading fanfic and analysing as I read. Sometimes you come across a technically bad story that makes you cringe, and yet there is something magical that draws you in. Conversely, you can get a 'beautifully' written story, written like it was composed by an English teacher - but it's dead.
This leads to the debate of whether good writing can be taught... You can definitely improve writing technique significantly, through learning specific tools and editing, editing, editing. But can you take someone who writes 'beautifully' but has no spark, and turn it into a blaze? Or to return to JK, can a Muggle who knows all the correct words, but has no magic, make a spell work—or can an untrained Wizard make magic?