According to Our Sunday Visitor Catholic Encyclopedia, those in the early Church who gave special witness to the faith were called "Father." In fact, St. Paul said, "For though you have countless guides in Christ, you do not have many fathers. For I became your father in Christ Jesus through the Gospel" (1 Corinthians 4:15-16).
The practice of addressing priests this way was brought to the U.S. in the 19th century by Irish immigrants who called their priests, "Father." Today this title reminds us that we are all united as family under Christ, his Gospel, his Church, and his representatives on earth.