Vilna Gaon, Malbim: It is because Hashem changed all the Mazalos. This was the primary miracle!

Rashi: This is why these days were fixed, and [the Megilah] was written, so coming generations will know.

Vilna Gaon: Refer to 9:26:3:6.

Malbim: This Igeres informed them everything in detail - how the miracle was in force on that day.

Torah Temimah citing Megilah 19a #1: The one who requires reading the entire Megilah explains 'why did Achashverosh use the Kelim of the Beis ha'Mikdash? Because he saw that Yisrael were not redeemed after 70 years. This is what happened to him, i.e. he killed Vashti.'

Megilah 19a #2: The one who requires reading from "Ish Yehudi" explains 'why did Mordechai provoke Haman? Because Haman made himself an idolatry. This is what happened, i.e. a miracle was done.'

Megilah 19a #3: The one who requires from "after these things" explains 'why did Haman envy all the Yisraelim? Because Mordechai wouldn't bow to him. This is what happened to him, i.e. he and his sons were hung.'

Megilah 19a #4: The one who requires from "on that night" explains 'why did Achashverosh ask to bring the chronicles? Because Esther invited Haman with him (15b). This is what happened, i.e. a miracle.'

Rashi Why did the people do these actions, and what happened to them. Why did Achashverosh use the Kelim of the Beis ha'Mikdash, and what happened to them? The Satan danced among them, and killed Vashti. Why did Haman envy Mordechai, and what happened to him? They hung him and his sons. Why did Mordechai not bow, and why did Esther invite Haman?

Vilna Gaon: There were three ¹ groups in Yisrael. The far ones did not know what was inside the letters only what was exposed (Divrei ha'Igeres - to be ready for Adar 13). Those closer saw what was inside - u'Mah Ra'u. The closest, their haters afflicted them - u'Mah Higi'a Aleihem.