I believe adiaireton8 is correct in saying that Mary gave birth to a unique being - a person with two natures, one of these natures which already pre-existed. But since the person is a combination of natures (via the hypostatic union), it is more proper to call Mary the Mother of God, rather than just the Mother of Christ ...
So then ... why wouldn't the scriptures do so ?
Certainly there was ample opportunity for the Apostles to include such a title in their writings ...
Quite honestly, our knowledge of God develops over time as we pore over His Word (whether oral or written). Did the Patriarchs understand everything God told them? I contend that the Apostles did not KNOW everything about God - and that the Spirit continues to reveal more about Himself through the Apostolic teachings already given.
Regards