I'll go with because I'd rather not read any spoilers for the book and because it'd be off-topic to mudbytes unless you're discussing a mud based on the series, though I was of the impression that J.K. Rowling was not willing to give permission for such a mud. :shrug:
Just because there are plenty of Potter MUDs out there doesn't mean Rowling authorized them all to be online. They're all running the risk of being shut down at any time. Assuming Rowling doesn't authorize this sort of thing.