Answer:
Monarchies
Why has monarchy gone into such precipitous decline over the last few centuries?
A rather rigorous class system must be recognized by the entire country for a monarchy to survive. The overwhelming majority of people must genuinely think that there is a class system and that those at the top are superior to those at the bottom. There shouldn't be upward mobility; some individuals should be born into working-class poverty while others should be born to inherit titles and rule over the working class. The Monarch, who is superior to everyone, is seated at the highest pinnacle of this structure.
A strict, universally embraced religious belief is added to this class structure. The Monarch reigned because God had given them the right to do so.
Societies have evolved over time to become more secular and egalitarian. People no longer think that someone with a title should have any privileges, thanks to industries like athletics, acting, music, comedy, etc. that allow someone from a working-class background to become quite affluent. In England, a person from a middle class family—with a teacher and a hairdresser as parents—can become a multimillionaire and knight by just fronting a rock and roll band and earning a knighthood.
In the meantime, religious belief has been slowly declining, and people no longer think a monarch was chosen by God.
In general, civilization has advanced past the point where a monarchy is justified. We are now adults. We think "that makes no sense at all" when we see someone in a position of power and influence only because of who their mother believes their father was. And we are correct.