Paris is called the "City of Light" because it was a place of enlightenment in the 18th century. During this age of enlightenment, Paris became the center of education, philosophy and learning.
Another reason Paris is called the "City of Light" is because it was one of the first cities to start using street lights during the Great Exhibition of 1889. Having street lights meant people could now do activities after dark that they could not do before. The streets suddenly grew safer. People were no longer forced to stay in their houses all night and when they did go out, they became more productive.