Because the Grimm brothers didn't make it into a feature film, that's why. And they didn't pay to copyright the story.
You're not going to like this, but that's how it works in the legal business world. Nobody gives a damn who came up with the idea, nobody cares who wrote the story, and nobody cares who put in all the blood sweat and tears. The legal owner of the story (or idea, whatever) is whoever filled out the paperwork and paid the fees to own the rights to it. The line had to be drawn somewhere, and that's where it was drawn.
Welcome to the world of reality. Nobody said it was pretty.