Thank you for your interesting question. I will try to answer the first part of your question first.
After the first sin of Adam and Eve, God promises that another woman will come to bring forth the Messiah. Genesis 3:15 states: “I will put enmity between you and the woman, and between your seed and her seed; he shall bruise your head, and you shall bruise his heel.” Later, through the prophet Isaiah, we learn that the woman will be a virgin and will give birth to a son who will save humankind. In Isaiah 7:14 we read: “Therefore the Lord himself shall give you a sign; behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.”
It is only in the New Testament that we learn her name. The Catechism of the Ukrainian Catholic Church, Christ Our Pascha, explains in paragraph 177 that:
This expectation of the Messiah and the drawing near to him are fulfilled in the person of the Virgin Mary from Nazareth. Before all ages she was chosen by God. Mary, who in accordance with Tradition was introduced into the temple as a young child, spiritually grew into the “handmaid of the Lord.” God progressively prepared her to become the womb, the ark, the temple, and the dwelling place for the uncontainable God. “By the good will of God the Father, the Most Holy Spirit prepared a dwelling-place in her for God the Word.” Mary became the tabernacle in which the Word of God came to dwell in the fullness of time: “The Word became flesh and lived among us” (Jn 1:14).
The Angel Gabriel greets Mary with a very unusual greeting: “Greetings, O favoured one, the Lord is with you! Blessed are you among women!” Luke 1:28 (NRSV). This, of course, is the basis for the Western Church’s prayer, “Hail, Mary, full of grace” and the Eastern Church’s prayer, “Rejoice, Mother of God, Virgin Mary, full of grace, the Lord is with you.”
That greeting gives us the answer to the first part of your question. Mary is full of grace and that is what makes her different from all other people. She said “Yes!” to the Lord. The evangelist Luke gives us an interesting contrast in the reactions of Zechariah and Mary when it was announced to them by the angel Gabriel that Zechariah’s wife would give birth to a son and Mary would conceive a son. Zechariah did not trust God and became mute. Only a person who is “Full of Grace” can accept God’s plan as Mary accepted it. God knows all His children who have ever lived and those who will be born to the end of the world. And because of that, God chose one woman, and made her “Full of Grace” to be able to give Jesus the perfect love in which he was immersed as a child. I hope you understand that Mary was chosen as a special woman to be the mother of Christ.
In my humble opinion, in order to address the other part of your question, I would have to speculate, and I don’t think I can do that. God chose Mary; He could have possibly chosen another woman, but then we would again find ourselves asking, why that woman and not another. I hope this gives you an answer to your question.
Fr. Mihajlo Planchak