Why is it that the Mona Lisa is so famous, out of interest?
Generally its because of the 'mysterious' feeling it gives off (e.g if you look closely, her eyes appear to stare back and follow you around) but in an art history sort of context, the piece's fame comes from its revolutionary nature. For example, it was one of the very first to use oil paint as medium, a pyramidal composition (wherein attention is directed towards an imaginary pyramid shape, giving the piece a sense of balance as a pyramid is considered balanced in art) and atmospheric perspective (things get hazier the further they are). Additionally, Da Vinci painted this during a time where almost all artwork was exclusively commissioned by the Catholic Church, so nothing was ever secular. The woman in the painting however was just somebody's wife (her name's Lisa Gherardini for those interested) and therefore not someone with any religious significance. This in turn also plays into the humanism movement that was going on back then where focus was shifted away from God and onto humans.