Final answer:
Friar John couldn't deliver the letter to Romeo because he was quarantined while visiting the sick.
Step-by-step explanation:
In Shakespeare's play Romeo and Juliet, Friar John could not deliver the letter to Romeo because he was quarantined while visiting the sick. In Act V, Scene 2, Friar John explains to Friar Laurence that he was held in a house suspected of having the plague, which prevented him from reaching Romeo on time.