On page 148 of her book, Joyce Little discusses God as Father. 'Now the God of Israel is Holy (Psalm 99). The word "holy" is rooted in the word "separated" (1 Chron. 23:13). God, being pure Spirit, has a certain "separation" or otherness to His material creation. This otherness of God is further revealed when He sent His only Son to the world instead of Himself. Even though we are created in His image, God the Father keeps His distance from matter to a certain extent. For this reason, the title "Mother" is not appropriate for God, since the words: "mother" and "matter", are etymologically related (Latin root: mater-). God is not Mother Nature or Mother Earth. Also mothers during pregnancy are biologically joined to their child, but fathers are physically separated. Even though fathers love their children, there is still a certain degree of distance as compared to mothers. Once again this "separation" of father from child is related to the "separation" (Holiness) of God from creation. The God of Israel is called Father not because He is male, but because He is Holy' " (from the online article, "Mother God" @ http://users.binary.net/polycarp/momgod.html).
The above excerpt is one of the best explanations I have encountered regarding why God is specifically called Father. Though the Bible often uses maternal images to impart understanding of the nature of God, it never calls him "Mother". Little's theology echoes that of George S. Montague in Our Father, Our Mother, with the idea that the male image of fatherhood reflects God's transcendence. While God is equally our Divine Mother, his official title, Father, is connotative of his separateness from creation; that is, we humans are not divine and neither is anything else in the material world. However, God does dwell among us in the 3rd Person of the Trinity, known as the Holy Spirit, who is often especially understood in bridal-maternal terms. But it is Mary, in her complete union with this Spirit of Wisdom, who best reflects the immanence of God's motherly nature. It is she who has been revealed, by Church Tradition through the Holy Spirit, with the titles Mother of God and Mother of the Church, to manifest for us an image of the sacred feminine by which we can most adequately experience God as daughter, bride, and mother.
Madonna of the Rosary - Murillo
Using the traditional language, Jesus' incarnation as the God-man can be understood most simply by virtue of his having a divine Father in heaven and a human Mother on earth, making him both fully human and fully divine. His existence as a human being illustrates that we can literally see our Father God in him, and also glimpse God as Mother through his own Mother, Mary, for she is the Mother of God. The way Mary loves Jesus is the way God loves all of his children like a mother. And she also represents the divinized state of being to which we aspire, when we too will inherit a glorified body and live eternally with God as his children. Mary reigns as our Mother, our Sister, our Queen, our Intercessor and Advocate. She is the Mediatrix of All Graces, working in complete harmony with the Holy Spirit of Wisdom who dwells within her to bring about our salvation, not in an equal way to her Son, but uniquely with him.
It still hurts sometimes, this overwhelmingly masculine language ascribed to God. Why is the motherhood of God so hidden? I believe it is because Wisdom, the Sophia at the heart of God, who is equally part and parcel of all Persons of the Trinity, is the Holy of Holies. Without Mary, we just can't get at this mystery at all. Without Mary, the Christian story is truly impoverished and lacking in the fullness of our beautiful faith. Brothers and sisters in Christ, fully embrace God as your Father, for it is a privilege granted to us by our Savior, and also fully embrace Mary as your Mother, for she is Jesus' sweet and holy Gift of Love. With Mary, our family is complete.