It's because her name is "Beauty." If her name were "Evelyn" ;), it would be "Evelyn and the Beast" or "The Beauty Named Evelyn and the Beast" or else just "The Beauty and the Beast."

Because one of the earliest & most influential English translations called it that. The original French is "La Belle et la Bete." But Andrew Lang dropped the first "The" in his version. Maybe he thought it sounded better. The heroine's nickname is "Beauty," so arguably, the title could be referring to her name rather than her description.