T.S. Eliot called the play Hamlet "the Mona Lisa of the literature world." Why? I think it is because when one looks at the Mona Lisa, it is difficult for some to see the fascination with it, and the same could be said of Hamlet, depending on who you ask.

When Eliot describes Hamlet as "the Mona Lisa of literature," what he means is that people think the play is a work of art because they find it interesting, rather than the other way around. In much the same way, many people regard the Mona Lisa as a great work of art because they are fascinated by the subject's mysterious smile. For Eliot, works of art need to be evaluated primarily as works of art; the question of whether or not they interest us is of secondary importance. And judging Hamlet purely as a work of art—irrespective of whether we find it interesting—Eliot finds it to be an artistic failure.
The main thrust of Eliot's criticism is that Hamlet's complex feelings don't find adequate expression in the play. According to Eliot's theory of the objective correlative, emotions cannot remain subjective, they must find expression in objects, a situation, or a chain of events. Hamlet fails in this regard, for Eliot, because there are no such elements in the play that correspond to the lead character's profound inner emotions. Eliot thinks that what we need in a work of art is something objective that will evoke an emotional state within us, something that will correspond indirectly to the emotion itself. If this isn't provided, then we're merely left with an excess of emotion without the prospect of its being fully articulated. And this, argues Eliot, is the main problem with Hamlet.
https://www.poetryfoundation.org/articles/69399/hamlet