The Great Gatsby by F. Scott Fitzgerald is a classic novel because it portrays in a brutally honest way how the human nature is fuelled by egoism and the need of constant validation. It also describes many socio-historical aspects of the United States in the 1920s: the development of the mass media, such as radio and tv; the mafia and the illegal activities; etc.
It has mantained a place in American literature for the quality of its content. It represents perfectly how the so-called and so-eager 'American Dream' is just a scam. Yes, Jay Gatsby did manage to earn money, power, a great house, but these things did not grant him real happiness: he did not have any real friends, people just pretended to like him for his position, and he was not loved back by Daisy.