I don't know why? It is an easy one. Mary was Immaculately Conceived for the sole purpose of being Christ's mother. Thus the cause of her sinlessness was Christ
I'll take a swing at it, but it's off the cuff (I'm really not a "Mary expert") but I think the point is educational regardless of whether it is directly on topic. Most of it is not uniquely Catholic and my description is somewhat annoyingly simplistic
Let's look all the way back in Salvation history: What would the state of the world be like if Adam and Eve had never fallen? We would merely be creatures (servants and slaves to be more exact). This would not be a bad thing of course - to spend our days walking in the garden with the Lord - but we would not have eternal life (or if we did, we would be more like the angels). Instead, we not only have eternal life in Heaven, we have become adopted sons and daughters of God. Heirs of God & co-heirs with Christ. We shall, in fact, reign with him throughout eternity. This "good news" is infinitely and immeasurably above what we were ever destined to prior to the fall (and why I believe that even the fall was part of God's design). So now we can say "Behold what manner of Love the Father has given unto us... That we should be called children of God... and so we are!" This is not "and so we are" as in "and son-of-a-gun we are", it is that He has declared us His children and He cannot declare a thing without it being so (God cannot lie).
But the mid-term result of their sin was to dis-grace us as a people. That is - Adam & Eve essentially said "no" to God and we became unable to ever say "yes" to Him. Our very nature is in opposition to His design for us and to His law. We have not the grace to respond to His call (until Christ, of course). We are generated after Adam and receive that fallen nature.
So now we come to Mary. Many non-Catholics (and many Catholics) believe that the Catholic position is that it would be impossible for a sinful woman to bear a sinless creature, so Mary had to be sinless. This is not so (and is demonstrably false since it would require Mary's mother to also be sinless etc. etc. etc.) Mary need not be sinless, and in fact, she certainly had committed the same types of sins all of us do. She only needs to be free from the dis-gracing nature of original sin (the stain of Adam) so that it is possible for her to say "yes" to Christ.
This does not mean that she had no personal sin from which she could to be saved. But more importantly, lacking original sin could only elevate her from renegade servant or rebellious creature to one who was not in opposition to God's will (at best, the level Adam & Eve started at). It would not make her bound for Heaven and eternal life with the Father. She must still be saved. Her simple "yes" may have been adequate (I'm not qualified to judge, but it seems that she had "received Christ" or "accepted Christ" in a unique way), but she certainly came to understand the things of Christ over the course of His life.
"It is by Grace you are saved through Faith and that not of yourself lest any man should boast."
This verse is often used to state the primacy of Faith (and the point is valid) but it is so often quoted (and often translated) as if the word "that" were not there. The point of course, is that even our faith is a gift of God's grace and not of our own doing. We retain our fallen nature when we come to Christ and are unable to say "yes" to Him by our own power - it is by His Grace.
I apologize for my somewhat unrefined explanation and I pray that I have not mis-stated the case.
Come Lord Jesus.