Why did absolute authoritarian countries almost disappear after World War II, with the majority of monarchies becoming constitutional monarchies?
This is a question worth pondering. Perhaps it is because the foundation of monarchism, the Divine Right of Kings, is not stable, as people are not as ignorant as they were in ancient times.
It may also be because most countries in Europe and America have adopted democratic republican systems after the two world wars. Under the strong cultural export and influence from these two regions, many former colonies followed Europe and the United States after gaining independence and adopted democratic systems. Republics emerged like spring bamboo shoots at this time.