What kind of slave? A human slave? They're more valuable in other forms of labor than harvesting their organs (usually; sometimes organ prices can exceed labor value). That's why slavery and organ harvesting still exist after all, if no one thought it was valuable, then it wouldn't exist. We don't have slaves in developed countries because we figured out that former slaves being able to use their brains for labor rather than their physical prowess was more valuable for society as a whole.