Because he is not infallible in everything. He is infallible only when he speaks in virtue of his supreme office as head of the Church on matters of faith and morals. He notifies us when he intends to define in accordance with all the conditions required for infallibility. This restriction to set occasions is as reasonable as the restriction of the jurisdiction of a civil judge to his official decisions in court.
Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty