Charles Ellicott Commentary John 19:21

"The chief priests of the Jews therefore said to Pilate, Write not, The King of the Jews; but that he said, I am King of the Jews." — John 19:21 (ASV)

Then said the chief priests of the Jews to Pilate.—Better, Therefore said ..., i.e., because the inscription could be read by everyone, and the Messianic title, “King of the Jews,” would be exposed to scorn. Yet these are the men who said, in order to accomplish the death of Jesus, We have no king but Caesar.

The expression, “chief priests of the Jews,” occurs only here in the New Testament, perhaps in contrast to the title, “King of the Jews,” to indicate that their anxiety about the title came from them as representatives of the national honour.

What I have written I have written.—The words are a formula to signify that the thing was done and could not be undone. There are frequent instances of similar expressions in the Rabbinical writings.