Why is Saint Monica the patron saint of mothers?

Monica was not a martyr or missionary. She was not a great teacher or preacher. She performed no miracles and has not been declared a Doctor of the Church. What she did, however, was to live a very holy and pious life and she prayed constantly for the conversion of her son, Augustine, who was leading a very sinful and dissolute life. She prayed for years and years. Finally her prayers were answered and Augustine was baptized and went on to become one of the greatest minds and theologians of the Catholic Church. What did Monica contribute to the Church? She gave us St. Augustine. Without her years of prayer we never would have had him.

St. Monica was not canonized as that system was not in place for hundreds of years after her death. She would have been declared a saint by the early Christian community based on her life of prayer.