Slavery in my story's world is common. Most don't see it as a good thing and slave-owners are rarely liked, but slaves are common enough nonetheless.
The royal family had personally stopped using slaves a few generations ago within their own employ, and instead have freed those slaves giving them the full rights of any other person.
That said, despite the royal family no longer using slaves, and instead only employing their workers, slavery still exists through the land. Typically, slaves are indicators of wealth, meaning you'd really only see slaves be owned by the upper and merchant classes. (So, basically, a minority of the population are slaveowners, not the majority.)
What could cause the royal family of a kingdom to personally abolish use of slaves, but be unable to outlaw the act altogether from within the kingdom? I'm not talking about making a world-wide decree that slavery is now illegal and enforcing it with brute strength. I am talking explicitly what could result in the fact the royal family can't even make their own merchants and middle-/upper-class obey a "no slavery"-policy.
The best answer explains why a royal family couldn't just outlaw the process altogether within its kingdom's borders, thus freeing the slaves.
If you think this question is still too story-based, please explain how so I can fix it. The previous piece of bolded text is the crux of what needs answered. Everything else is just the skin placed over the story.