God chose Mary for multiple reasons:
1) Because of God’s undoubted knowledge and foreknowledge of her personal and spiritual qualities, and of her ability to raise the Son of God and to successfully endure the trials that she would experience as His mother as a result of opposition to Him (starting when He was two years old or less, all the way up to His crucifixion);
2) Because of her descent from David (along with Joseph) in fulfillment of prophecy (starting with 2 Samuel 7:16), but (unlike Joseph’s descent in Matthew 1:2-16) being descended from David through Nathan (rather than Solomon) (as indicated by the genealogy in Luke 3:23-38), thus avoiding the curse placed on Jeconiah — also known as Coniah — and his descendants by God in Jeremiah 22:30;
3) Because (in God’s foreknowledge) of the fact that she lived at a time when she would be required to travel to Bethlehem (where Micah 5:2 had indicated that the Messiah was to be born) just before giving birth as a result of the census ordered by Augustus, and because of her betrothal to a descendant of David. (1 came to pass in those days , that there went out a decree from Caesar Augustus that all the world should be taxed. 2 This was the first enrollment made when Quirinius was governor of Syria.);
4) Because she lived in Nazareth, to which she and Joseph would return to raise Jesus, which would allow ultimate fulfillment of Isaiah 11:1;
5) Because she was a virgin (in fulfillment of Isaiah 7:14), and yet was betrothed to Joseph, which would allow Jesus to have an earthly father. Perhaps Mary was the only individual who even possibly could have satisfied all these requirements.
6) Mary also “found favour with God”, not her merit or works of holiness or righteousness.