Because the film is a very complex movie, focusing upon the seven samurai in question, thus showing us different sides of our own human nature. It's got it all - action, drama, comedy, and a deep look into the human condition. Plus it was made by, arguably, the greatest Japanese film director EVER, Akira Kurosawa, and almost every film that he made was a brilliant work (only in his later years did his films begin to suffer). Great acting, great directing, and most of all, a very great story equals an all around classic film. It's THE perfect samurai flick.
Then again, I'm a big Kurosawa fan. Seven Samurai is my personal favorite of his films, alongside Ikiru, which is an extremely powerful movie in it's own right.
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Thank you, WWFallon! You are my Olympic Hero!
"Being a bastard WORKS."
--Spider Jerusalem