Phantom of the Roller Coaster: Part 2 - S3-E24
Question: Why did Wonder Woman make Randy forget where he was the past few days?
Answer: Because he knew that Diana Prince was Wonder Woman and watched her in action. Since he had been missing for a couple of days, she needed to erase everything he knew.
Couldn't she just make him forget that Diana and Wonder Woman were the same but leave everything else?
Answer: She also erased all the secrets and mysteries surrounding the amusement park.