Why? Well, really think for a moment about this line from an ancient hymn: “He whom the entire universe could not contain was contained within your womb, O Theotokos.” Catholics and Orthodox tend not to think about that much because they take it so much for granted. It's like: "Yeah, sure, what else is new?" But taking such a reality for granted is just plain silly, at least from the standpoint of faith. We have here what should be a constant source of meditation and inspiration. For that purpose, I shall say two things.
First, as a callow young conservative I learned to love that line because it is so very un-PC. It reminds us of how Christianity differs from other religions. As Pope John Paul the Great wrote:
Jesus was born of the Chosen People, in fulfilment of the promise made to Abraham and constantly recalled by the Prophets. The latter spoke in God's name and in his place. The economy of the Old Testament, in fact, was essentially ordered to preparing and proclaiming the coming of Christ, the Redeemer of the universe, and of his Messianic Kingdom. The books of the Old Covenant are thus a permanent witness to a careful divine pedagogy. In Christ this pedagogy achieves its purpose: Jesus does not in fact merely speak "in the name of God" like the Prophets, but he is God himself speaking in his Eternal Word made flesh. Here we touch upon the essential point by which Christianity differs from all the other religions, by which man's search for God has been expressed from earliest times. Christianity has its starting-point in the Incarnation of the Word. Here, it is not simply a case of man seeking God, but of God who comes in Person to speak to man of himself and to show him the path by which he may be reached. This is what is proclaimed in the prologue of John's Gospel: "No one has ever seen God; the only Son, who is in the bosom of the Father, he has made him known" (1:18). The Incarnate Word is thus the fulfillment of the yearning present in all the religions of mankind: this fulfillment is brought about by God himself and transcends all human expectations. It is the mystery of grace.
Second, this distinctive mark of Christianity, the Incarnate Word, is also a paradox: "He whom the whole universe could not contain..." God brought us eternal life by dying as a man; but given the purpose of his dying as a man, he had to be born as a man. As the living, active receptacle of the paradox of the Incarnation, the Virgin Mary is herself part of it: Mother of her Creator. She didn't understand the thing when it was first announced to her; but never doubting it, she never withheld her cooperation.
Meditating on such facts is no mere pastime for those who have little else to worry about. By experience, I learn that the littleness of God in the Christ Child is what brings his greatness to me and strengthens me when all else fails. The littleness of Mary is also what enabled her to become the greatest among us. Starting as the humble maidservant by whom the Creator of the world entered the world to save it from itself, she continues to bring him to us by her prayer, intercession, and appearances. She as great power over that Adversary whose power in the Church can sometimes seem to approach his power in the world. In all such ways, she is the one through whom her divine Son now saves the Church from the Church. Very timely, I should think.