Here is my guess, which I have never read anywhere. My unique interpretation! lol
Ok. When the Holy Spirit came down and everyone heard the preaching of the Gospel in their own language, I think that the same thing happened when Jesus wrote in the sand. It was not recorded what he wrote because the apostles discussed it afterward and one said ‘he wrote this…’ and another said ‘no, he wrote that’, and they could not agree. What they knew was that it was unique to their own personal situation, their own failings. Jesus wrote one word in the sand and everyone saw something different. An arrow hit straight to their hearts. They could not speak, they walked away, and pondered that moment of truth and light in their hearts.
No one wants to broadcast that they saw ‘thief’, ‘adulterer’, ‘liar’… Then they were angry and looked for a way to kill him.