See I blame George Lucas for this.
Because he did the prequel trilogy, people became obsessed with making movies explaining all the backstories of this that and the other.
Why does Maleficent need two backstory movies? Why does Hannibal Lecter need a backstory movie? Why does the Father Merrin need a backstory? Why does Dracula need a backstory movie? Why does Nurse Rached need a backstory? In most cases, we either already know everything that we need to know to make the story work OR the character works best when shrouded in mystery and its best to not pull the curtain back.
Hell, my biggest problem with GI Joe Rise of Cobra was that the crux of the story hinged on Cobra Commander's backstory....the character who thrives best when he's shrouded in mystery and you know almost nothing about it.
What is this obsession with having all the questions answered? Doesn't that, oh, I don't know, kill the mystique and take the fun out of alot of these things? Or in the case of Nurse Rached, just beat a dead horse because we already know everything that we need to know?
That isn't to say prequels can't work. The Planet of the Apes trilogy was very good, and Bates Motel worked much better than I expected it too, but as a general rule, the less you know, the better.