God cares for this world so He chooses people
The bible tells us that He predetermined that role of the prophets before they were conceived. Isaiah 49.5 and Jeremiah 1:5.
In a special way, Mary was chosen for the Salvation of humankind and therefore she received the privilege of being conceived free from the stain of original sin. Pope Pius IX says she received the benefits of salvation in Christ from the very moment of her Conception because God chose her to be the Mother of the Saviour. Read more