Uncle Tom's Cabin perhaps had a dramatic effect on American public opinion about slavery because it was a work of fiction. There had been many Northern accounts of life under slavery, ranging from abolitionist tracts to travel narratives like those of Frederick Law Olmsted. These had persuaded many Americans that slavery was a moral wrong. But by using the medium of the novel, Stowe allowed herself license to develop characters in ways that could arouse sympathies in the reader. Popular literature in the nineteenth century tended to be highly sentimental, and Uncle Tom's Cabin was squarely in this tradition. By pointing out the effects, physical and moral, of slavery on highly sympathetic characters that readers could identify with, she highlighted the evils of slavery in a way that other writers may have found it difficult to do. It is also true that Uncle Tom's Cabin was written as a popular novel, written deliberately to gain a wide readership, and people, then as now, simply read more of that kind of material than non-fiction. So Uncle Tom's Cabin, as a novel, reached a wider readership than other genres could.
One must also remember when the novel was published. The book came out in 1852, at a time when Northern public opinion was beginning to turn against slavery in any case. The Fugitive Slave Act of 1850, passed by Congress as part of a compromise over the issues raised by California's admission to the Union, essentially required Northerners to surrender fugitive slaves to slavecatchers. This was deeply unpopular among Northerners, and brought the issue home in a way that little else could have done. Many Northerners were beginning to imagine slavery as a national sin, and the vivid scenes and somewhat stereotypical characters portrayed in Uncle Tom's Cabin reinforced that feeling.