I loved the painting before I ever knew it was stolen.
I think it’s fair to say there is something about this painting that isn’t in many other works or most other works, and it’s a human and psychological quality more than some special painting technique.
But is that because the painting was already famous and well known because of the theft (despite your not knowing about the theft)? That is, if you were a few hundreds years old and showed in your journals how you loved the painting in the decades before it was stolen, that would be significant. But saying you loved the painting before you know it had ever been stolen, but falling in love with it in the decades after it became famous because of the theft doesn't mean as much, does it?
The argument seems to be that I fell in love with the Mona Lisa as a result of it being famous, versus some other quality it has which I find appealing.
But for example, Van Gogh’s Sunflowers is also incredibly famous but I don’t find that painting appealing at all. For me, it’s boring. Ditto for many other famous paintings. What I’m saying is that I’m not sure so sure that’s it’s logical to say: “oh this thing is just popular/well loved because it’s famous, there’s no other reason”
Anyway like what you like and ignore the nerds :)
Throw enough stuff at the wall, eventually something sticks.
Do it often enough, and something becomes iconic.
And even if it was only slightly different from its peers at the time, that one thing is what will be remembered.
It's not a mysteriously unknown technique, or of particular historical importance, or the only painting by da Vinci. It become famous mostly because of the theft incident and once you're famous the famousness tends to feed itself because people do things like X-ray it which adds to the mystique. It was owned by some famous people including Napoleon and it's spent a lot of time in art museums being shown to the public, but there's no secret message on the back or alien decoder ring or demascus steel/roman concrete "how'd he do it?!" mystery.