Another main reason: There are so many excellent masterpieces and poems in French literature, which make people feel fragrant when they play. There are many works that describe the scenery, people's delicate psychology and society: Hugo, Merimee, Balzac, Mo Bosang, Roman Roland, Dumas and Rousseau ...
Of course, there are other factors that make people believe in the beauty of French: painting art, opera, philosophy and so on.
French is strict in grammar and clear in expression, which also makes people admire it.