Answer: For a significant part of the Church's history, powerful monarchs and governments influenced the choice of popes. (See Secular Influence section in Wikipedia article on Papal Conclave) St. Pius X in Commissum nobis (Jan. 20, 1904) absolutely abolished the secular veto.

References
• Make Aquilina and Grace Aquilina, Chapter 81: A Ballot from The Conclave That Elected Pius X, A History of the Church in 100 Objects, Ave Maria Press, copyright 2017
• Papal Conclave Wikipedia https://en.wikipedia.org/wiki/Papal_conclave
• Veto Power In Papal Elections, Catholic Encyclopedia, https://www.catholic.org/encyclopedia/view.php?id=10047



Pope Pius X Beatification
Scott # 145-148 (1951)


Pope Pius X Canonization
Scott # 182-184 (1954)