""There's a same sort of "policy" for the German Holocaust. So why not for this?""
Because the Germans themselves did not end the Holocaust. The Allies defeated the Nazis, bringing the Holocaust to an end.
The British on the other hand, while not even starting slavery, became the leading force in trying to stamp it out.
Still, personally of the opinion that the history of slavery be still put someplace close to "front and center" and not shoved aside with "that was in the past--time to move on" so as to prevent such a (as the article aptly put it) crime against humanity cropping up again, and stopping it where it still exists.
From an historical standpoint, rather than an emotional or propagandistic standpoint, the west should be congratulated for ending slavery, not villified. It's interesting to me at least that more attention is paid to events hundreds of years ago - when as a point of fact slavery is still practiced today. As a practical matter maybe that is where energies should be focused.