Ever since the 9th century, the Supreme Pontiff of the Roman Catholic Church has been crowned with a tiara at a special coronation ceremony. According to the Vatican, "the Papal Tiara is formed by three crowns symbolizing the triple power of the Pope: father of kings, governor of the world and Vicar of Christ."
Paul VI received the tiara in a special coronation ceremony, but was the last pope to wear the it.
On November 13, 1964, Pope Paul VI, donated his jewel-covered gold and silver three-tired tiara to the poor at a ceremony at St. Peter’s Basilica. It is said that the pope was inspired after hearing about the poor around the world during the Second Vatican Council.
When John Paul I was elected pope, he would have been crowned with the tiara, as there was no other ceremony. However, he asked his master of ceremonies to develop an "inauguration Mass" that did not include a coronation.
This act was in accord with John Paul I's official motto, Humility.
He explained in his second and last homily, at the Possession of the Chair of the Bishop of Rome, that he desired to rule the Church in the example of St. Gregory the Great.
Subsequent popes always have the option of reinstating the tiara, but none has done so, no longer seeking to be seen as a temporal ruler, but only as a humble pastor.