Rashi’s explanation notwithstanding, Parah Adumah remains a Chukah. The Jews sinned, they sullied themselves. What did they do to deserve Hashem’s kindness that the Shechina should purify them?
What did the baby do to deserve its mother’s kindness? It sullies itself again and again and again and it is lovingly cleaned up countless times by its mother. Can you explain a mother’s love? There is no logical rational for the mitzvah of Parah. Hashem purifies His people because he loves them.
On Purim Hashem displayed his love for his children. By preparing their salvation while they were sinning by party of Achashveirosh with the killing of Vashti, Hashem conveyed the message, “I’m with you in the depth of this exile. I’m taking care of you even though you have degenerated to the point of being expelled from holiness.”
Parshas Parah continues the process of Purim. “I come to be with you even when you are in the depth of filth. And I help you even further; I will purify you and bring you into the circle of holiness because Pesach is coming and I want you reclining by my side at the Seder.”