It makes sense. In order for books about slavery as something wrong to sell, they need to pander to the white audience. White people going back and forth between pro-slavery and anti-slavery won’t read a book where the main characters don’t relate to them in some way. Hence why in both The Heroic Slave and Uncle Tom’s Cabin, the main characters are well-educated, attractive, and kind in nature. They are also often their masters favorite or well liked by their masters.
In The Heroic Slave, Madison Washington is described as handsome and kind. He can read and is a skilled hard worker. In Uncle Tom’s Cabin, Eliza is a mulatto, she appears very white and is a sweet woman who is well-educated. She is described as follows: “There was the same rich, full, dark eye, with its long lashes; the same ripples of silky black hair. The brown of her complexion gave way on the cheek to a perceptible flush, which deepened as she saw the gaze of the strange man fixed upon her in bold and undisguised admiration. Her dress was of the neatest possible fit, and set off to advantage her finely moulded shape;—a delicately formed hand and a trim foot and ankle were items of appearance that did not escape the quick eye of the trader, well used to run up at a glance the points of a fine female article” (Stowe Chapter 1). Eliza is attractive and relatable. A slave the white people could feel comfortable with.
It’s clear that both authors use slave characters that pander to white audiences to help their stories reach a broader audience. When people don’t feel threatened by the black character of a book they may even end up liking the character. While it may not feel like the right way to represent the slaves and how they should be freed, it makes sense as a strategy to even get your cause out there.