There is a simple reason: The United States does not yet have the stomach to look over its shoulder and stare directly at the evil on which this great country stands.
Hmm - could be true (sometimes I think this myself), but also kind of unfalsifiable, not really grounded in anything concrete. But, still "keep reading" ...
That is why slavery is not well taught in our schools.
What? It damn sure was in my school. Along with the evils of Reconstruction, Jim Crow, lynchings, and general trajectory of events leading to the civil rights movement. And the eugenics movement, and Indian Removal. And several lectures devoted to the practice and evils of racism as such (and as applied to other group). Along with questions about whether slavery really ended at at all after the Emancipation Proclamation, or just changed form. Yeah, we went there.
Still, I don't know from nationwide tendencies, and who knows what the author means by "well", so keep reading ...
That is why any mention of slavery is rendered as the shameful act of a smattering of Southern plantation owners.
WTF? Absolutely not true, in my lifelong observation. I'm sure it happens, but in an extreme minority of mentions. Essentially every time the topic has come up, it has always been described as an "institution", a systematic evil, absolutely embedded in the fabric of life in the Southern states (and respected by the legal frameworks of Northern states, etc).
There are definitely legitimate questions to ask about whether mainstream America (or even "White America") has sufficiently examined its racist and genocidal past. And yes, about how this shapes policy and debate up until the current day.
But where this author thinks they're going with this train of hyperbole, I don't know - and won't be able to stick around to find out.