The miracles we celebrate on other holidays were overt miracles, therefore we say Hallel on those days. Purim, on the other hand, was a hidden miracle and therefore we say a hidden “Hallel” (i.e. Megillat Esther). Purim as a day of kabbalat HaTorah.
Rabbi Taub: Purim, Hallel, & Kabbalat HaTorah
February 17, 2013