Probably for the same reason that you sanitize the history of the Founders being traitors to their government when they rebelled.
And apparently your condemnation of slave owners convniently ignores George Washington, Thomas Jefferson, Madison, Monroe and the majority of early Presidents.
Or do you condemn them as well?
Did those men start a war o overthrow the constitution as the supreme law of the land? Did they believe slavery was a moral good and blacks wete ordained by God to be slaves?
Lets look at what those founding fathers thought of slavery;
I can only say that there is not a man living who wishes more sincerely than I do to see a plan adopted for the abolition of slavery.
George Washington letter to Robert Morris 12 April 1786
The abolition of domestic slavery is the great object of desire in those colonies, where it was unhappily introduced in their infant state.
Thomas Jefferson
Our opinions agree as to the evil; moral, political, and economical, of slavery.
James Madison
“What was the origin of our slave population? The evil commenced when we were in our Colonial state, but acts were passed by our Colonial Legislature, prohibiting the importation, of more slaves, into the Colony. These were rejected by the Crown.”
James Monroe
Now lets look at what the southern rebel leaders thought of slavery;
“African slavery, as it exists in the United States, is a moral, a social, and a political blessing.”
~Davis
Our new government is founded upon exactly the opposite ideas; its foundations are laid, its cornerstone rests, upon the great truth that the negro is not equal to the white man; that slavery, subordination to the superior race, is his natural and normal condition.
Alexander Stephens Vice President of the CSA
I think any reasonable person can see the difference in the viewpoints on slavery by the founding fathers of America compares to the founding fathers of the confederacy.
The difference is that the founders all wrote and advocated for freeing slaves. Democrats political platform was keeping slavery.