[advice4u]

Which only makes it more remarkable that its popularity continues to grow. That fact suggests there is a special timelessness to the film, since as you say most classics are either prototypical, unconventional, or artistically innovative (like, as was mentioned, Citizen Kane).

The closest analogy I can think of while remaining in the realm of the arts is Vermeer, who was a fairly typical Dutch genre painter unrecognized in his time and ignored for two centuries. Nowadays he is popular and beloved, but unlike Rembrandt who is held as a great master or van Gogh who is held as a quixotic genius, Vermeer himself is taken as wholly mundane; his works simply have some "je ne sais quoi" that resonates with the average person.