It’s famous for precisely the same reason that the works of Shakespeare, Mozart, Homer et al are famous. They’ve been adjudged the pinnacle of their respective disciplines.
And if the Mona Lisa is a “modest creation” I’d love to know what the questioner considers a superlative one.
That seems to be a circular argument. Why is the Mona Lisa judged to be the pinnacle of its form?
Why is Monty Python considered so great?
Why is it ok to snub other comedians like Larry the Cable guy?
Getting back to paintings, its just a picture of a woman. I’ve seen other paintings depicting battle scenes and such that are not just huge but show great detail.
The Mona Lisa is famous for her smile – while posing she was riding Da Vinci’s much younger rival Michelangelo.
But what, exactly, makes it so perfect?
Your answer is no answer at all.
Was that a whoosh?
For the simple reason that it shows a human being in a multidiminsional and happy state. It resonates deeply in the viewer.
Doesn’t resonate with me. I find it utterly tedious.
And I doubt Muffin’s Michelangelo theory, since Mike was supposedly as gay as a tree full of parrots. But I doubt that, too, since he sculpted David’s and painted Adam’s dicks so small. 
I’m not sure myself why the Mona Lisa is as famous as it is.
For my money, the Last Supper and Creation of Adam are far more superlative and unforgettable Renaissance works.
If we started from zero and tried to chose “the best painting”, even if we started with only 50 of the most famous, pre-19th century ones, I don’t think Mona Lisa would win. For all it’s technical aspects and influence it’s not a painting the stirs the soul per se; even the famous smile would not be as revered.
There is an aspect of “informed excellence” in it.
The robbery was key and now it would almost be impossible to get a new one as “the best”.
Small penises were standard in Graeco-Roman sculptural design, so you can’t really draw any conclusion from that.
Enigmatic: difficult to interpret or understand; mysterious.
Chiaroscuro: an effect of contrasted light and shadow created by light falling unevenly or from a particular direction on something.
Analogous Color: Analogous colors are groups of three colors that are next to each other on the color wheel, with one being the dominant color, which tends to be a primary or secondary color, and one on either side of the color.
Earth Tones: is a color scheme that draws from a color palette of browns, tans, warm grays, and greens. The colors in an earth tone scheme are muted and flat in an emulation of the natural colors found in dirt, moss, trees and rocks.
Also, and I don’t know the exact terms but, like most portraits Mona is placed in the center of the portrait to grab your attention. There is also contrast between foreground and background, her, and then the rocks and hills fading into the background. Also, her face and hands form two more focal points, they are positioned - just - far enough apart that you more or less have to center on only one point at a time, face/smile, background, or hands.
Also, and I forget the name of the term, but the paint/oil was applied in many shallow layers, adding to the mysterious effect. Plus, the scenery, you can’t quite place “where” she is, that looks like “normal” hills and trees but it has a surreal effect to the geography as well.
And then of course, is her expression. Muted and subtle but conveying or implying…
But really, it is the mystery, the enigmatic effect. Movies like Mullhoand Drive or Under The Skin, movies where the exact meaning is unclear, are often times much more captivating, arresting, then simple and easy to understand movies.
