Because the franchise is too big, and makes far too much money for Bloomsbury and Rowling to walk away from.
It’s naïve to believe that they would throw away the considerable investments made in both Harry Potter and JK Rowling. The 8th Potter book, if ever written, will sell like hotcakes – as will the first book in a new series “by JK Rowling, the author of the Harry Potter series”.
If I were Ladbrokes, I’d open books on both, the 8th Potter and the first non-Potter Rowling.
And I hope they would acknowledge the fact that they got the idea here.