Answer:
because he knows Oedipus is the cause for all the trouble in Thebes.
Step-by-step explanation:
At the beginning of Oedipus King, there is a plague ravaging Thebes because the murderer of the former king has not been caught. Oedipus summons Tiresias, a blind prophet, for help. Tiresias knows the answers to all of Oedipus questions, but he refuses to speak them because he also knows that it is Oedipus himself who has caused the plague by unknowingly murdering his father and marrying his own mother.