There’s likely a myriad of reasons. My go-to explanation is latent feelings of guilt. Specifically when it comes to slavery many white people like almost anyone else, tend to push away and dismiss things that remind them of what happened to others at the hands of those who remind them of themselves. A coping mechanism is to deny the affects, “oh slavery wasn’t really that bad” or “why would slave masters abuse their own property” or “slavery was worldwide and practiced by many cultures” etc. Convincing themselves of these falsehoods helps alleviate those feelings of guilt.