This came up in some random askreddit thread and while I feel like I can guess at some reasons, and there's some Quora answer out there addressing it, I'd really be interested in a historian's answer.
They did. The moral, legal, and racial/white supremacy justification for race-based slavery in the United States initially began with the enslavement of Native Americans, and those same justifications were then applied to support African slavery. Native American slaves were ubiquitous throughout the Americas, from coastal New England to the coast of Patagonia, and from Columbus's first voyage to California slaving raids after the Civil War. Check out this previous answer for more information.
Also, Andrés Reséndez The Other Slavery: The Uncovered Story of Indian Enslavement in America is a wonderful, vital, and very readable introduction to Native American slavery. I highly recommend this book.