Taxes are raised to cover the excessive spending that had taken place under the past administration and to keep a balanced budget. The greatness of Mordecai is recorded in the history of the kingdom. We are also told that he was 2nd in command.
Tracing God's Fingerprints in the Legacy of Peace
Esther 10:1
And the king Ahasuerus laid a tribute upon the land, and upon the isles of the sea.
The island of Tyre and Cyprus were under the control of the Persians and so taxes were raised on the entire land mass including the islands.
Esther 10:2
And all the acts of his power and of his might, and the declaration of the greatness of Mordecai, whereunto the king advanced him, are they not written in the book of the chronicles of the kings of Media and Persia?
Because of the many coincidences in Esther, some have considered it "just a story," having little or no historical validity. Yet as archaeologists have learned more about ancient Persian history, the details of Esther have proven remarkably reliable. Certainly the book presents itself as history: here the author confidently asserts that readers can check the facts in the historical records. These records are lost, but evidently they were known to the early readers of Esther. (Compton’s Complete Interactive Bible)
Esther 10:3
For Mordecai the Jew was next unto king Ahasuerus, and great among the Jews, and accepted of the multitude of his brethren, seeking the wealth of his people, and speaking peace to all his seed
God’s name never appears in the book but I see him on every page─his plan─his action─God leaves us spellbound─his fingerprints are all over this story. God doesn’t take any credit─he just get the job done.