Why Only Mary Preserved from Original Sin?

The simple answer is that this was a key part of God’s eternal plan for humanity. Jesus is the “new Adam” (Romans 5:12-21) who frees us from original sin and our personal sins, and Mary, as the early Church Fathers would say, is the “new Eve” in cooperating with God’s plan in this regard.

And yet God does not leave anyone orphaned in sending his Son as our Redeemer (see John 3:16-17). In addition, though the Blessed Mother was immaculately conceived, she still had to cooperate with God’s grace in carrying out her life mission.