During the age of enlightenment in the 18th century, Paris was the center of education, philosophy and learning. This was one reason it was called the city of light.
Paris was one of the first cities to use street lights. Street lights allowed people to do their activities after dark and so Paris was called the city of light.
Paris was the first city in Europe to use street lights. In 1828, Paris began lighting the Champs-Elysées with gas lamps and so it was called 'La Ville–Lumière' or the city of light. See why Paris is called the city of love.
According to one theory, Paris got its name when the authorities asked people in the 18th century to put lighting candles or oil lamps on their windows to reduce the crime rate.
In modern day, Paris is a heavily lit city. It has over 300 lit sites, many heavily lit streets, in addition to the The Eiffel Tower which is lit at night. While this was not the reason Paris got its name, it could still be one reason why Paris retained that name.