Question

Why doesn't Juliet tell Nurse about her plan to escape the wedding

Answer 1

Because the nurse is her trusted adviser, and basically like another mom to Juliet. ;)

Answer 2

Answer: Because the nurse might tell the Capulet family.

Explanation:

The nurse was sincere about preference towards Paris. In fact, the nurse told Juliet that she had to marry him because he is a gentleman. If Juliet told the truth to the nurse, the whole Capulet family about her plan and the wedding would be interrupted.