Ending the book with Ahashverosh's greatness puts the story in perspective. This is the story of Ahashverosh and his kingdom - not a Jewish story. Mordekhai's greatness is entirely dependent on the king's whims. This viewpoint is reinforced by the many allusions to the story of Joseph throughout the Book of Esther: Joseph rose to the highest power imaginable, but after his death the Jews were enslaved in Egypt.