The answer to this can be found in the preface to the book  that William Lloyd Garrison wrote.  Basically, he says that they are unable to believe what they hear.

He says that people do understand that slaves are held as property.  But they do not understand or cannot comprehend how bad that truly is.  So they think that they people who say how bad slavery really is are just making it up.

He says that the people who hear the stories think the people telling the stories are just telling lies to make the slaveowners look bad.  He says the people

affect to be greatly indignant at such enormous exaggerations, such wholesale misstatements, such abominable libels on the character of the southern planters!