Because Purim is the Holiday of "V'Nahapoch hu," of Topsy Turvy and Upside Down, of evil plans falling upon their plotters' heads. It is the Holiday which shows our enemies with their high and mighty plans for world domination, for being Number 1, winding up at the bottom of the list!
Of Haman wanting to ride on the King's horse, in the King's robes and crown, pulling Mordechai through the streets of Shushan on that horse wearing that clothing!
To paraphrase a once-popular tune about a certain Lola, "Whatever Haman wants, Mordechai gets!"
And this is the one Holiday that we are permitted to laugh at our enemies! To mock them and to make up funny plays about them! As it says in "Mishlei"/Proverbs 3:34, "As concerning mockers, He will mock them."
When the mighty general Sisera was killed by a woman, Yael, Devorah the Judge and Prophetess sang about that event, "…so may they be destroyed, all the enemies of Hashem! And as for those who love Him, may they be like the sun rising in its glory!" ("Shoftim"/Judges 5:31)
On this Holiday of Purim, we show our faith in Hashem that the bigger our enemies, the harder will they fall! "Utzu etzah, V'Tufar, Daberu Davar V'lo Yakum, Ki Imanu E-l," "Make your plans; they will be nullified - Hold your conferences of hate against us, and all your plans will not succeed; for Hashem is with us!"