Let’s keep in mind the pasuk in Zecharyah (8,19):
כה אמר ה’ צב-אות צום הרביעי וצום החמישי וצום השביעי וצום העשירי יהיה לבית יהודה לששון ולשמחה ולמועדים טובים האמת
והשלום אהבו
All the fasts, 17th of Tamuz, 9th of Av, Tzom Gedalyah the 3rd of Tisri, 10th Teves will be converted to the Jews for joy and happiness and yomim tovim provided if truth and peace is loved. Taanis Esther is not mentioned as that shows the result of prayer, the taanis was designated later and this taanis is for happiness.