Why? I would think it quite the other way, that according to the Bible ALL have sinned... Therefore, the likelihood is that Mary did too.
Knowing God has this power and knowing Mary would be the mother of Jesus, why would Jesus not honor His mother by creating her this way?
This is speculative, and can just as easily be turned about: Why, necessarily, would he do so? In fact, since ALL have sinned, wouldn't it be considerate of the Scriptures to suppose He did not?
Furthermore, the very understanding of Original Sin being a lack of a relationship with God becomes muddled if we are to consider a woman with God in her womb who simultaneously lacks a relationship with God. It does not follow.
And why not? Her relationship with God is defined as such, but that need not assume her to be sinless in any part of it.
When we say created "without sin" we mean without Original Sin at the moment of her creation. The same as Eve.
(And you are placing way too much literalness on the Scripture "ALL have sinned." Little babies and the retarded, for example, do not sin. So not all have committed personal sin.)
This is speculative, and can just as easily be turned about: Why, necessarily, would he do so?
How about because He had to follow the Commandments perfectly and one of them states you should honor your mother (and father)?