In William Shakespeare's "Romeo and Juliet," the Friar's letter fails to reach Romeo due to a series of unfortunate events. Specifically, Friar John, who is tasked with delivering the letter to Romeo, is unable to complete his mission because he is quarantined. The text reveals that Friar John is unable to enter Mantua, where Romeo is, due to a plague outbreak in the area.
Evidence from the text suggests that the authorities prevent Friar John from leaving the city because they fear he may spread the plague. He explains this when he says, "I could not send it—here it is again—Nor get a messenger to bring it thee, / So fearful were they of infection." This quarantine directly contributes to the tragic miscommunication that ultimately leads to the demise of both Romeo and Juliet.
Thus, the failure of the letter to reach Romeo is a result of external circumstances beyond anyone’s control, particularly the fear of disease that engulfs the community.