Why is the Mona Lisa so popular?

Mona Lisa is so famous because the world has various guesses on who that mystery woman was that Leonardo had painted. She's actually Leonardo's Chinese peasant mother, Catrina.

reasons for her fame is the 'Mona Lisa's Smile'. Da Vinci exploited human optimcal illusion to create a unique smile through perspective and by using shadow work. Da Vinci painted Mona Lisa in such a way that the eyes are the center of the viewer's attention and the mouth is the periphery.