So, out in Den Colorado many slavers make profits from buying and selling human slaves. The often prey on small tribes and settlements, like Boulder and Five Families. The slavers ride into town, take some people, and then leave, with those people captured subjected to unimaginable suffering as a slave. There is one thing in my plot that doesn’t make sense, however. What would be a plausible reason why the people in these towns wouldn’t try to fight back?
Combination of factors.
1: They stand to profit by selling their own criminals and captured enemies or assorted other undesirables to the slavers.
2: They own slaves, and a good number of those slaves come from..... you guessed it. The slavers.
3: Cultures change over time. We consider slavery reprehensible now (not enough to stop using Cambodian toddlers to make all our stuff but I digress). Owning slaves is entirely accepted in the post apocalyptic culture. It's a profitable way to build large things cheaply in with little to no industrial capability.
4: Slavers are respected, working for them is a very profitable opportunity everyone aspires to, its a fast track to wealth and power.
5: The slavers are smarter than one dimensional cartoon villains and don't mindlessly attack allies cuz "WE SLAVERS! WE BAD! FIGHT YOU NOW!" They operate like the Colombian narco-lords. They provide services and charity to their people, they act as a de facto police and government in a time where there is little or no social cohesion. Slavery becomes the punishment for most serious crimes, giving a crude sort of justice system to the wasteland. They make allies, cultivate friendships, do brisk business, and always ensure their neighbors have the right balance of fear, respect, admiration, and dependency to never think of attacking. But lastly and most importantly, they cant sell slaves to people if they get a reputation as mindless violent idiots and kill or enslave potential customers.
In the early days of Africans being taken into slavery by Europeans, often the local West African rulers would sell their prisoners of war (rather than their own people) to the slavers. http://www.nationalarchives.gov.uk/pathways/blackhistory/africa_caribbean/africa_trade.htm
The people of Boulder & Five Families might (whether openly or under the table) use the slavers as a way to get rid of their "undesirables" -- petty criminals, social rejects, etc.
People pay protection money to the slavers. Those who don't pay are the ones collected.
People don't need to fight back as long as they are paid up......
Like every sort of criminal organization, a protection racket is a great source of income with a low level of work required.
The slavers hit the town taking a few every so often. After awhile someone comes into town offering protection from the slavers for a price.
Eventually, those who don't pay up are actively targeted for slavery. The remaining townsfolk feel it's cheaper to pay than fight as the slavers have stated they kill burn the town and kill everyone that resists and enslave everyone else should they try and rebel.
If you're set on the idea of slave raids (as opposed to the slavers buying or extorting people from their settlements), consider making them stealthier. Slavers are basically predators, after all, and when falling on herds predators always single out the weak and isolated. Now weak victims don't make all that useful of slaves, so logic dictates they'll go after people who are alone or in small groups - tending the outlying fields, say, or on journeys to other villages. Each settlement has a fortified core that can repel slaver raids, but sooner or later somebody has to take a chance and venture further out.
Alternatively, what if they bring back indentured servitude? In a society like this, most people are going to be spending most of their time performing manual labor anyway, so doing it for someone else isn't that big of a change, and it can be safer working under the thumb of a domineering landlord than taking your chances as a subsistence farmer. Depending on the circumstances, your lot in life might actually improve. ("It sucks having to do what Mr. Johnson says, but at least there's clean water and no dire coyotes.") There's no need for raids and corrals and guards if the slaves volunteered.