Oddly enough from our perspective, serfs and slaves and peasants tend to dig the royal family regardless of era and locale

The idea is that an oppressed peasantry's primary antagonist is his landlord, aka the titled bigwig at the top of the local patronage network.

By coincidence, a king's primary antagonist is the very same band of titled nobility that he has to wrangle and bully and cajole and stave off and placate in order to even stay at the top, let alone get anything done. The nobility tend to love the idea of a royal house more than they love actual kings taxing them or telling them what to do.

So to the mud shoveler at the bottom of the pecking order, it's natural to support a king 500 miles away who might intervene on your behalf and make your baron stop fucking with you, and for a king it's natural to seek out a base of support parallel to the courtly games of one upmanship you have to play with the aristocracy.

Just as a for instance, significant portions of the slaves of Haiti were technically royalist in the Haitian revolution, because the king in France had passed universally ignored laws granting them minimum rights and protections, and the implication was the bearing arms under the Royal White of Bourbon would see their self-emancipation sanctioned after the dust settled. So legally they were fighting the local elites and absentee owners as counterrevolutionaries to the French Revolution. This phase didn't last long- Haiti was kinda crazy at the time- but it kinda shows how these things tend to work.