The title stems from a legitimate tale as old as time: "La Belle et la Bête."
Translation? You guessed it: "The Beauty and the Beast."
Why? Familiarity, mainly. Movie titles serve as marketing first and foremost, and you don't want to confuse the audience by misrepresenting your product. Everyone knows that this is the name of the fairy tale, and by making it the title of the film, we know exactly what we're getting into.
As a counterexample, you can look at the more recent Disney movie Tangled, which is really a retelling of the story of Rapunzel. Why didn't they just call it Rapunzel? As with Beauty and the Beast, it comes down to marketing. While Beauty is a very traditional fairy tale, Tangled is a little more satirical, going for a screwball comedy feel over straight-up romance. Disney felt that would appeal to boys and teenagers, who might be scared away if they thought it was just another princess story.
It all comes down to the Benjamins, even with something as simple as a tale from a storybook.