These things actually happen in the real world, there are many examples and many reasons.
Religion might be used to keep people at bay.
People might just simply not know any better. If that is the system they had always known, and possibly in the past or somewhere else it is/was even worse, they'll be ok with it. If in the past people were also enslaved or beaten up, it won't seem too bad to simply have to work for a poor pay.
Brainwashing: you can make people believe it is good. And make them believe that outside, in all other places, it is worse. Whether that it is true or not.
You can find examples of this literally everywhere around the world.
In some countries there are codified classes, and people keep accepting it, for religious/cultural reasons.
In some countries there are monarchies, and people seem ok with being inferior to the royals.
In most religions women are considered inferior. Even just in the Catholic Church. Yet churches keep on being full of women, who seem ok with the idea. If you internalise some concepts since you're a kid, you won't question them too much.
In USA, most people would benefit from public health system and public education. Yet among the poorest classes you'll find millions who defend a system that goes completely in their disadvantage, just because that is what they have been taught to believe.
In most Western countries people who can barely make ends meet will pay money to go to see the new illiterate reality-show star showing up in a club for half an hour. Is that different from accepting that you are underpaid, while some other people just deserve more, and even being happy about it?
You really do not have to invent too much to create a reality where the lowest classes will happily defend the oppressing system.