We are actually never told why Creon is late returning from his visit to the oracle at Delphi. Creon never makes an excuse for his lateness, nor does Oedipus ever raise the issue again once he points out to the priest that Creon is taking longer to return than Oedipus would expect, as we see in his line, "... And already enough time has passed that I wonder what he is doing, for he has stayed beyond the proper time" (78-80). However, we do learn that Creon's tardiness helps give rise to Oedipus's suspicions that Creon is conspiring to overthrow him. Therefore, we can also assume that Oedipus believes Creon's tardiness has to do with his treasonous plot, although we later learn that Creon is completely innocent of any plot against Oedipus.
Oedipus first begins to suspect Creon of treason when Creon informs him that the late King Laius was killed by bandits. Oedipus finds it hard to believe that bandits, or a bandit as Oedipus suggests, would kill a king unless they were paid to do so by some external party, as we see in Oedipus's lines, "How did a bandit come to dare so much, unless he acted with money from here" (135-136). Therefore, Oedipus suspects a conspiracy to overthrow the crown of Thebes has been afoot since Laius's time. Next, after Tiresias delivers his horrible prophecy, Oedipus believes that Creon has paid Tiresias to deliver false prophecy in an effort to drive Oedipus away so that Creon can have the crown. However, Creon denies all accusations of treason and even remains faithful to Oedipus after he has had his great fall. Creon will not even assume power as king until he has consulted the gods for advice.
Hence, while we never learn why Creon is late, we certainly learn that his lateness had nothing to do with treasonous plots against Oedipus.