Given that the Harry Potter films have been immeasurably successful, it’s no surprise that Warner Bros. would like to continue capitalizing on the success of the property despite the fact that the story has come to a conclusion.  Solution?  Spin-off.  Warner is producing a series of films based on the fictional textbook (which was later turned into a real book) Fantastic Beasts and Where to Find Them.  The spin-offs will not be prequels, per-se, but it sounds as though the first film will begin in New York 70 years before the events of The Sorcerer’s Stone and is intended to expand the fictional universe.  And make cash.  That’s probably the real reason behind this.