How is it that Romeo and Juliet is part of the modern canon?

Romeo and Juliet contains a universality of thought and feeling that transcends time, culture, and political thought; therefore, I would argue it should remain part of the modern canon.

Works of art developed in a medieval Italian city can affect us too. What does this require? . . . That these feelings and moods shall have received such broad, intense, powerful expression as to have raised them above the limitations of the life of those days. 

Interestingly, these words are those one of the first leaders of Marxist Communism, Leon Trotsky. He argued in 1924 against a number of Soviet writers who contended that the "reactionary culture" of the past should be excluded from their "new society." 
Literature is the recording of the human heart; therefore, it is timeless. As long as human beings walk the earth, there will exist among them antipathy, erotic desires and love, generational problems, and emotional and psychological weaknesses. Indeed, William Shakespeare's Romeo and Juliet, a tragedy of two "star-crossed lovers," includes all of these thematic traits found universally in people. So, because of these universal elements in this work of literature, it is as relevant today as it was during the Elizabethan Age.
When, for instance, Friar Laurence counsels Romeo— 

These violent delights have violent endsAnd in their triumph die, like fire and powderWhich as they kiss consume (Act II, Scene 6, lines 9-11)

—these words could issue from any parent or spiritual adviser of any time or place. The universality of literature transcends any age, culture, and race because it is the recording of the human experience.