"Beauty and the Beast" is the name of a traditional fairy tale. Why isn't it "The Beauty and the Beast"? I don't know. In French it is. In English it isn't. It has been known as "Beauty and the Beast" in English for at least 250 years, so the name remains.
The title character's name, Belle, is French for Beauty so the English title can be read in two ways - the girl Beauty and the concept/ideal of Beauty. In most English language versions of the story, the character's name is actually given as Beauty not Belle. I think the Disney version (much more recent than my childhood) has made the Belle version popular.