On the simple level, the Malbim writes that the book was written “before the king” – Achashverosh. His supervision of the writing guaranteed that the incident was preserved in its completeness and accuracy, without any misinterpretation or even negative spin a human scribe is prone to add.
On a deeper level, the Alshich, Vilna Gaon, and the Shaar Bas Rabbim all point out that a regular book can be burnt or can disappear. Therefore the “king” before Whom this was written was H-Shem, the King of kings. His direct Supervision of this event was a reward for Mordechai and Esther’s self-sacrifice.