Let me explain why God made Mary sinless. There are two things the Church understands. For one in Biblical typology, there types of Mary in the OT.
Let me pull one such part. The Old Covenant is type of Christ because Christ’s blood is the Blood of the New Covenant.
The Ark of the Old Covenant contains within it the Ten Commandments, Manna, and the Aaron’s rod which symbolizes priesthood. The Ark is made of Acacia wood which cannot be decayed easily. It is also made of beaten gold.
The Ark of the Covenant is type of Mary because Mary is the literal Ark of the Covenant. When Jesus came into the world, Jesus remained inside Mary’s womb for nine months. Jesus we know is the Word of God, he is also the Living Manna, and the he is the Priest.
Just as the Ark of the Covenant is pure. Mary is also pure. You know in Scripture, that anyone who touches the Ark dies.
Joseph, as Jew would understand this and did not have sex with Mary after Jesus was born. It is through Mary’s flesh that Jesus have his own flesh from.
If Mary had sinned throughout her lifetime, her flesh would be impure and unworthy to be the Mother of God. You know God is very much offended by sin. He does not want his Son, Jesus to born of some woman who is with sin.
So God created Mary free of original sin. He made Mary just like the way he made Eve before the fall. To say that she is has sin, you would claim that God is not capable of making Mary sinless.
I say God is capable of making Mary sinless. He is the Creator.
I also like to add that the Early Church Fathers believed that Mary is the Ark of the New Covenant.
St. Anathasius of Alexandria (c. 296–373) was the main defender of the deity of Christ against the second-century heretics. He wrote: “O noble Virgin, truly you are greater than any other greatness. For who is your equal in greatness, O dwelling place of God the Word? To whom among all creatures shall I compare you, O Virgin? You are greater than them all O [Ark of the] Covenant, clothed with purity instead of gold! You are the ark in which is found the golden vessel containing the true manna, that is, the flesh in which divinity resides” (Homily of the Papyrus of Turin).
I would carefully read this article:
catholic.com/thisrock/2005/0510fea5.asp
and this article:
cts.org.au/1998/overshadowingtheshadows.htm