Because it acknowledges the ugly side of American history, and conservatives still think that the Charlie Brown specials are an accurate representation of how this country was founded.
This isn't accurate. Most people are well aware of the evils of slavery, discrimination, Japanese-American internment, the murder of Indians and the theft of their land, and the other big crimes in American history. If you ever interacted with any sizable amount of real people from conservative states and communities then you'd know that.
MacArthur is right that CRT is vague but what probably motivates people against it above all else is the belief (right or wrong) that it implies that they are
personally implicated in this history simply for reaping any of its benefits.
