When anti-slavery northerners tried to outlaw native slavery in the west, White southerners rallied in defense, because they knew that if native slavery was outlawed, it was the lead domino to fall in a process that would threaten plantation slavery in the South. So they build this transcontinental defense of un-freedom, because they knew one un-free institution depended on the other. And they were successful, un-free Indian labor wasn't outlawed before the Civil War. In fact in took decades after the war and 13th Amendment for Indian slavery to be stamped out in the West.