Abolition of monarchy - Monarchy has been in a steep decline. Before recent centuries, nearly every nation-state was a monarchy, and some monarchies were very long-lived, even if not very continuous, like the Pharaonic and Chinese monarchies. Some republics became monarchies, like the Roman Republic and the Dutch Republic, with the stadholder becoming a de facto monarch.
Making the US a republic was a very daring move, and it helped that George Washington didn't want to make himself King George I. France became a republic in its big revolution, but it did so by chopping off the king's head. The revolutionaries squabbled and reorganized and chopped off a lot of other people's heads, and they became a bad advertisement for republicanism. As a result, nation-builders in European nations wanted monarchs for their new nations. Holland, Belgium, Norway, Italy, Serbia, Albania, Greece, Romania, Bulgaria.
This trend was interrupted by World War I, a war which got in the way of an aristocrat going to Finland and becoming that nation's monarch. That war also ended four big monarchies, both directly and indirectly, the Prussian/German one, the Austrian one, the Russian one, and the Ottoman one. Most of the nations that emerged from that war had no monarch, though Hungary didn't have one because Hungarians were split on whether to have a Habsburg or a local aristocrat as monarch.
European monarchs in Italy and Greece fell from association with disliked dictators, and those of Bulgaria, Romania, Yugoslavia, and Albania fell from Communist takeovers.
Looking elsewhere in the world, when most of Latin America became independent in the early 19th cy., the revolutionaries chose republics for their new nations, with the exception of Brazil. That nation's monarchy fell late in that century.
Most of the nations that became independent in post-WWII decolonization became independent as republics, with the exception of Middle Eastern and North African nations. But some of those nations also became republics.