When Jorge Bergoglio visited the United States, his trip made headlines, just like the visits of Karol Wojtyla and Giovanni Montini. Bergoglio is better known as Pope Francis; John Paul II and Paul VI were his predecessors.
But why do cardinals change their names when they are elected pope? There is no strict doctrine requiring it. For centuries men assuming the top job in the Roman Catholic Church kept their birth names.
The first person to adopt a new moniker was Pope John II in 533. Born Mercurio, he felt it unwise to have the name of a pagan god while serving as pontiff. His successors went back to the old way of doing things until Pietro Canepanova was chosen pope in 983. He did not want to use the name of St. Peter, the first pope, and so switched to John XIV. He was followed by Giovanni di Gallina Alba, who retained his birth name and became John XV. After his death in 956, popes began using an alias, known as a regnal name, on a regular basis. The last pope to use his real name was Adrian VI in 1522 (he was also the last non-Italian elected until 1978).
The regnal name is frequently chosen with a purpose. Bergoglio selected Francis in honor of St. Francis of Assisi, foreshadowing his commitment to the poor. Albino Luciani, following after John XXIII and Paul VI, dubbed himself John Paul. He died after a 33-day reign. Wojtyla styled himself John Paul II in his memory.
The most popular name has been John, used by 21 men, even though the most recent was John XXIII. There have been 16 Gregorys and 15 Benedicts. Francis is one of 44 popes to have a unique name.