Many whites in the North supported slavery because they didn't understand the reality of what it was like for the enslaved. Many whites believed the ideology that the slaves were childlike creatures who were happy and well cared for by benign owners. Many whites, going along living their daily lives, didn't stop to imagine what it would be like to be owned by another person and subject to all their whims, with cruel punishments for disobedience. Many people didn't stop to note that sometimes the owner was a child, without a fully developed conscience, who was given a slave as a "gift."

Therefore, books that publicized and detailed the lives of slaves were important to changing hearts and minds. Former slave Frederick Douglass, for example, was at pains to counter arguments often used to justify slavery. For example, the seemingly happy singing of the slaves was often used to argue they were content with their lot: Douglass argued that the songs emerged from deep anguish over being held captive. He also dispelled the notion that slaves who said their masters treated them well were speaking the truth, telling the story of a slave who mistakenly complained about his master, only to be sold south to the cotton plantations.

Other whites, like Harriet Beecher Stowe, believed slavery would gradually wither and disappear and so did not worry too much about opposing it. It was only when politics seemed to be moving in the direction of strengthening slavery that Stowe rebelled and wrote Uncle Tom's Cabin. This book created a firestorm in favor of freeing the slaves immediately, showing that many people's support of slavery came from a lack of imagining what it was really like, rather than active belief in the institution.

First of all, we need to note that we cannot know for sure why so many whites supported the institution of slavery.  There were no opinion polls or other ways to scientifically determine what attitudes people held.  Therefore, historians have to make educated guesses.

Historians tend to make two arguments in this regard.  First, they say that many of the Southerners who did not own slaves aspired to do so someday.  This is similar to why people today who are not rich do not call for socialism.  People today do not want to tear down the rich because they hope to be rich someday.  Similarly, poorer whites in the South hoped to be rich enough to own slaves and did not hate the institution itself.

Second, historians say that slavery was good for the self-esteem of the poorer Southerners.  Southern whites who were poor could still feel good about themselves because of slavery.  They could know that, no matter how poor and degraded they were, they were white and that made them better than the slaves.  This led them to want to keep slavery.