It took God many centuries to create a woman pure enough to become the mother of God the Son incarnate. She was descended from many of the holiest people. God wouldn't just allow any woman to give birth to Him--His humanity came from His mother. Remember that Christ had a full human nature--otherwise we could not be saved. Only His human mother could provide Him with the human nature. If Mary had declined to do God's will, how many more centuries would it have taken to produce another woman pure enough to provide the human nature for Christ? It's something to think about.