I am in the middle of King Leopold's Ghost, which I'm reading in part because it seems the Congo has become a yardstick for Europeans' inhumanity to Africans during the colonial period (as well as their ability to ignore and forget atrocities). Is it really an outlier, though? Didn't South Africa and Cuba both develop forerunners to "concentration camps" around this time under England and Spain, respectively? I'm sure there were other events that showed European excesses. Why is it that the Belgian Congo seems to get mentioned more? Was it really worse, or is it just a newly (re)discovered horror?
Historiography can't really answer value judgements. It's not our place nor expertise to say which occurence was more horrifying. However, you could say that the atrocities commited in the Congo Free State - which is not the same as Belgian Congo -, were somewhat out of place for that day and age. These atrocities did continue to some extent in the Belgian Congo, but the conditions of the native population significantly improved once the Belgian state took control.
I can try to explain as to why these atrocities seem to be refered to so often though. Belgium was a very new nation at this point in time. It only became a country in 1830 and mostly only because other nations were sick of fighting and didn't mind a neutral zone in between them. So they joined the "Scramble for Afrika" very late and were very eager to also start making some profit of the rich resources of Afrika. I say "they", but this was mostly the project of one man and a very small elite. There was some notable outcry in Belgium aswell about the conditions of the natives, and these conditions significantly improved once the colonial rule was transitioned over to the Belgian state.
That being said, while Leopold II his methods were particularly crude, his intentions were not completely out of sync with the attitude of the ruling elite in Belgium at that point in time. I've done some research about Belgian politics around this period and it's very striking how much "self-assertion" is present within their policies. This often led to very progressive attitudes and political changes, but it might have also affected Leopold II in this instance. He was extremely eager to improve and glorify the Belgian state and to glorify his own person, so much so that he did it at the cost of the native population of Congo. It also has to be said that these atrocities are fairly well-documented and often served as an example.
Why did it serve as an example? This part is just a hypothesis, but I'd like to add it anyway. I believe that when this new player, who had no idea on how to run a colony, commited such atrocities, yet acted from the same paternalistic and supremacist perspective as others nations had towards their colonies, it forced the international community and other nations to take a good look at their own motives and attitudes.