Why do you think the nurse puts off giving Juliet the news when she knows Juliet has been waiting? What does this scene reveal about their...

This scene is for comic relief, but it also shows Juliet and the nurse have a close and loving relationship. 

The reason the Nurse puts Juliet off is because she knows Juliet is waiting. She realizes how important marriage is to an impatient teenage girl, and she is teasing Juliet, milking the moment for all it is worth. The scene is very funny, so it creates comic relief for the audience between many dramatic and important scenes. Additionally, it shows what a close relationship the two have.  

Nurse

I am a-weary, give me leave awhile:
Fie, how my bones ache! what a jaunt have I had!

JULIET

I would thou hadst my bones, and I thy news:
Nay, come, I pray thee, speak; good, good nurse, speak.

Nurse

Jesu, what haste? can you not stay awhile?
Do you not see that I am out of breath? (Act II, Scene 5) 

We know Juliet is not close with either her father or mother. She was basically raised by her Nurse. Nurse is charismatic, bawdy, and genuine. She teases Juliet because she cares about her. She knows about Romeo. This means Juliet trusted her with this secret information.  

JULIET

Here's such a coil! come, what says Romeo?

Nurse

Have you got leave to go to shrift to-day?

JULIET

I have.

Nurse

Then hie you hence to Friar Laurence' cell;
There stays a husband to make you a wife. . . (Act II, Scene 5)

Since Nurse is one of the few people who knows about Romeo and Juliet, she is also one of the few who understands why Juliet can’t marry Paris.