Today is the Solemnity of the Immaculate Conception. Not Jesus’ conception. That has its own special day (the Annunciation). Today is about Mary.
One of the (many) problems caused by sin is how it messes with us. Not only does sin make it harder to know what to choose. Sin makes it harder for us to understand the consequences of our choices.
Which is part of the “why” of the Immaculate Conception. God had a huge ask for Mary, and God wanted her to be able to see her choice clearly. To fully understand all that it would mean. Including the heartbreak of Good Friday.
The Immaculate Conception sets the stage for the first truly free choice since the Fall. The even greater miracle of Mary’s “yes.”