Asking "why" questions in theology is always a bit of a guessing game.
Why is the senior God called "Father"? And why is the mediator called "Son"? In what sense are we the "children" of God? And where is a father and a son without a mother? A craggy old Mormon apostle in 1878 stated:
“What,” says one, “do you mean we should understand that Deity consists of man and woman?” Most certainly I do. If I believe anything that God has ever said about himself ... I must believe that deity consists of man and woman ... there can be no God except he is composed of the man and woman united, and there is not in all the eternities that exist, or ever will be a God in any other way.”
In Genesis, the account has God saying, "Man has become as one of us." Genesis also states that "God" made man in his image, after his likeness. In these verses, the word for God is not Jehovah, nor is it God. The word is Elohim, the plural form of God. So if the Gods created man in their image, after their likeness, male and female, wouldn't that imply that if men are in the image of God, and women were in the image of God, that God is comprised of both male and female?
Methodist scholar Dr. Margaret Barker has just written a fascinating new book entitled The Mother of the Lord, Vol. 1: The Lady in the Temple. In it, she traces the ancient Israelite tradition that God has a consort. The review states: "Margaret Barker traces the roots of the devotion to Mary as Mother of the Lord back to the Old Testament and the first temple in Jerusalem. The evidence is consistent over more than a millennium: there had been a female deity in Israel, the Mother figure in the Royal cult, who had been abandoned about 600 BCE. She was almost written out of the Hebrew text, almost excluded from the canon. This first of two volumes traces the history of the Lady in the Temple, and looks forward to the second volume in which Barker will show how the Lady of the Temple is reclaimed in the advent of Christianity, and becomes the Lady in the Church. The result is breathtaking, and like all Barker's work, is impossible to put down."
In her introduction, she writes: "A vast spectrum of material points to this pattern of events, and in this first volume I shall present evidence for the lost Lady of the first temple and for her first exile. She was removed from texts by the work of ancient scribes and the assumptions of biblical scholars. In the second volume I shall show how she survived during the time of the second temple, and how her temple was finally restored by the Christians."
Barker also has documented the ancient Israelite concept that Yahweh (Jehovah) is not the Father God, but was the firstborn of the Father God and was the God of Israel. In other words, Jehovah is not Michael the Archangel, but is the premortal Jesus Christ.