Why is classical music used in ballet?

For the following reasons: * It was the music of the time when ballet was first being danced. Classical music was the "popular" and "cool" music of the time. * It's beautiful songs really connect with ballet. * Even after classical music was not the popular music of the day, everyone thought of ballet with classsical music. * Older ballets were already written to classical music, it wouldn't make sense to rewrite the music for those historic and beautiful ballets.