For reasons known only to Him.
Also, it is very dangerous to say He was waiting for someone like Mary. That gives credence to the Protestant belief that we worship Mary or view her as something great independant from God. He chose the time and the place and then chose to make a woman, whom happened to be Mary, into what she is. It is through God alone that she is who and how she is. God created the goodness in her, He did not wait for somebody to come around who was good enough, because NOBODY in and of themselves would have been good enough, Mary or otherwise.