IMO it's because, to their minds, unofficial fanfic threatens both their livelihood and their claim to be professional writers. And in some cases, that's entirely true. I don't feel any desire at all to read Buffy pro-fic because my impression is that the best of the fanfic is so much better. The exception would be where fanfic writers turned to pro-fic. But you know, if they'd just write better novels, they wouldn't need to feel threatened.