Why was Mary chosen to be the mother of Jesus?

The Bible simply states that she had found favor with God.

There is a general theological and narrative trajectory in the Scriptures of God's preferential option for the least, the last, and the lost. In Genesis, the stories of Abraham (a wandering nomad...a nobody), Isaac (the second born of Abraham), Jacob (the second born of Isaac), and Joseph (11th of Jacob's sons) reveal a pattern that would have been quite unusual: it was usually the eldest son who received the primary portion of the inheritance, but in these stories the "least" become the greatest. The Old Testament presents a preferential option for the least and those without power as God's modus operandi when it comes to Israel, which, politically speaking, is generally much weaker than the surrounding nations. Consider the story of the Exodus, in which Israel is set free from mighty Egypt, and the accounts of the return from exile in Babylon and Assyria: through these God lifts up Israel, the least of all the nations, as the light for all the nations.

Fast forward to the Christian telling of the birth of Jesus. Quickly after the resurrection of Jesus, Christians, most of whom are Jews, see in their experience of Jesus' death at the hands of Rome a fulfillment of Israel's story. Jesus, a lowly carpenter, is made to triumph over Rome and even death itself by the power of God. Christians came to view Jesus as the ultimate sign that God would bring justice to the poor and oppressed and they identified in his life and teaching the fulfillment of God's promises.

The earliest narratives of Jesus are written in such a way that they highlight this theme of God's preference for the least (see Luke 4:18-19). There is a strong contrast painted by all four gospel writers (Matthew and Luke, in particular) between real power and authority (God's) and the prevailing power and authority of the day (Rome). So, when it comes to the birth of the Savior of the world, the logical vessel of this salvation would be a mighty ruler, a king, or a rich person. But, not so for God. God chooses to work through one of the lowliest people imaginable...an umarried peasant girl from a little off-the-map town in a province that the most powerful empire in the world looked on with great scorn (to further underscore this theme, Luke tells that Jesus was born in a stable and was countenanced by shepherds and foreigners). That's why God chose Mary to bear Jesus. The good news (gospel) and the narrative unity of the story of the people of God would lose its integrity if Jesus had been born as a rich guy.