Because decrees, once issued, could not be undone.
Remember, after Haman was hanged, Esther asks Achashveirosh to call back the decree. He doesn’t do it. He gives Esther the ring and says that anything that is sealed with the King’s signet ring cannot be recalled (meaning the original decree cannot be recalled – not necessarily the new one). So instead of recalling the decree, he tells her to write as she wishes to the Jews. Esther tells them to defend themselves.