The African slave trade lasted nearly 400 years. It's unbelievable, isn't it? It took that long for serious thinkers to realise that the practice was inhumane. Mind you, rebellions from the slaves themselves did act as a big prompt that they might have something to complain about.
And even if the voice of slaves had little impact, the voice of a female author Harriet Stowe alerted wives and daughters of the rich landowners, raised their sensibilitities and sympathies and hastened the end of slavery. The book was Uncle Tom's Cabin, and powerful stuff it was too. Nothing like a well written novel to alert the literate to unfairnesses.