Good question!

I think part of it has to do with hegemonic ideological power. To quote myself:

“Antonio Gramsci, the Italian Marxist theorist, coined the term “cultural hegemony” (although ideological hegemony also works) as the idea that the ruling class imposes the prevailing norms on the rest of society, which are then believed to be natural, inevitable, benevolent, etc. 

This makes revolution more difficult, because those oppressed by the system don’t yet see their suffering as injustice (as opposed to bad luck, or the will of God, etc.) and can’t imagine a world organized differently than it is. Hence why Gramsci argued that intellectual liberation was necessary for political liberation, or why E.P Thompson argued that class is a process of people creating a new world-view (rather than just a result of material forces). 

In a post a while back, I linked this idea to Steven Lukes’ idea of the three faces of power. Lukes talked about the three faces of power as decision-making power (formal state power), agenda-setting (the ability to decide what’s within the realm of legitimate debate, what is considered a “problem” and what isn’t), and ideological power (the ability to influence other people’s thinking, even when that thinking is against their interests). For example, we can see the third face of power in the fact that, even though Wat Tyler had seized London, he still felt that he needed King Richard to give the commons a charter of liberty and trusted that the King would keep his word that he would issue one and his word that Wat Tyler would not be harmed during a parlay.”

So if the dominant ideological framework of your culture is that kings are chosen by God and that rebellion against them is a sin, you need a lot of ideological lifting power to get people who are being actively oppressed by the monarchy to rise up against it. Hence why so many peasant rebellions from 1381 to the Bauernkrieg drew on religious justifications for rebellion, because the only thing above the king is God. 

But an even bigger ideological lift than that is trying to envision an alternative method for organizing political authority if one was actually to succeed in overthrowing the system, especially if there aren’t multiple ideological frameworks available to your culture/time. While monarchy might not be a very good system, like primogeniture it had the advantage of being relatively simple and having fairly universal acceptance. This made it superior than the alternative of chaotic civil war among rival nobles who no longer have any central authority to check them.

And even when there are other models, transitioning isn’t exactly easy, as we can see from the history of revolutions. Not only are other models usually denigrated by the existing culture. political systems require no small amount of learning.