From catholic.com:
Mary: “Full of Grace”
The Fathers of the Church taught that Mary received a number of distinctive blessings in order to make her a more fitting mother for Christ and the prototypical Christian (follower of Christ). These blessings included her role as the New Eve (corresponding to Christ’s role as the New Adam), her Immaculate Conception, her spiritual motherhood of all Christians, and her Assumption into heaven. These gifts were given to her by God’s grace. She did not earn them, but she possessed them nonetheless.
The key to understanding all these graces is Mary’s role as the New Eve, which the Fathers proclaimed so forcefully. Because she is the New Eve, she, like the New Adam, was born immaculate, just as the First Adam and Eve were created immaculate. Because she is the New Eve, she is mother of the New Humanity (Christians), just as the first Eve was the mother of humanity. And, because she is the New Eve, she shares the fate of the New Adam. Whereas the First Adam and Eve died and went to dust, the New Adam and Eve were lifted up physically into heaven.
Of particular interest in the following quotations from the Fathers are those that speak of Mary’s immaculate nature. We will all one day be rendered immaculate (sinless), but Mary, as the prototypical Christian, received this grace early. God granted her freedom from sin to make her a fitting mother for his Son.
Even before the terms “original sin” and “immaculate conception” had been defined, early passages imply the doctrines. Many works mention that Mary gave birth to Jesus without pain. But pain in childbearing is part of the penalty of original sin (Gen. 3:16). Thus, Mary could not have been under that penalty. By God’s grace, she was immaculate in anticipation of her Son’s redemptive death on the cross. The Church therefore describes Mary as “the most excellent fruit of redemption” (CCC 508).
All Mary needed was the right lineage and to be a virgin.
That was ALL that qualified her for the job.