Act 4 Scene 1 why does Juliet tell her nurse to leave her room?

Because the Nurse is the most plausible person to enter her bedroom and wake her up. You note in Act II it is the nurse who supervises the bedroom, whose calls Juliet answers from her balcony. Again in Act III it is the Nurse who alerts Juliet that her mother is on her way, and Romeo had better scoot. Her parents never enter unless the Nurse has seen her previously.