Why is Barracoon so polemic?

Barracoon: The Story of the Last Slave by Zora Neale Hurston is an unsparing portrayal of the horrors of slavery through the eyes of one of its victims, Oluale Kossola, the last survivor of the last known slave ship to sail from Africa to America. In telling Kossola's story, Hurston doesn't pull any punches in her depiction of slavery and the immense suffering it caused.
This accounts for the relentlessly polemical tone of the book. Hurston isn't simply allowing Kossola to tell his story here—although she most certainly is doing that, faithfully transcribing his words in their original dialect—she's also using the old former slave to make a general point about the annihilation of African tribal culture by white slave-traders. Prior to being sold into slavery, Kossola witnessed at first hand the destruction of his community, in which a rival tribe was complicit. Unlike more traditional historians, Hurston makes no effort to adopt a disinterested perspective, to stand back from her sources and observe the story they reveal with scientific detachment. On the contrary, this is history as polemic, an unashamedly ideological take on a dark chapter in American history.