One uses specifically the ashes of a heifer, not a bull, in order to remove the impurity imparted by a corpse. The Sages compare this to a mother who cleans the filth generated by her baby.

Why are all the offerings male while this one is female? Rabbi Aivu said: This can be explained by means of a parable of the son of a maidservant who sullied the palace of a king. The king said: Let his mother come and clean the excrement. So said the Holy One, Blessed be He: Let the heifer [i.e., the mother cow] come and atone for the sin of the calf. (Bemidbar Rabba 19)