You only posted half of the title.

Declaration of the Trafficking of Enslaved Africans and Racialized Chattel Enslavement of Africans as the Gravest Crime against Humanity

The abstaining countries mostly has a Problem with “the gravest crime against humanity”, because there should be no ranking in crimes against humanity.

Where do you place the Holocaust, the holodomor, the crusades? The conquest of the americas?

Side thing, but I don’t see the Crusades as at the same level of the Holocaust or the Holodomor. They were religious wars of conquest not campaigns of extermination. They were brutal, sure, but if you add them, then you have to start piling a bunch of other wars in there too, like the Mongol conquests, the Arab conquests, the Ottoman conquests etc. Which kind of dilutes the point of “grave crimes”.

There is nothing particularly unique about the Crusades, and at the time, the Roman Empire that invited them and tried to sanction them actually had a legitimate claim of them being reconquests of Roman territory.

The crusades involved kicking muslims and jews out of the land. It was definitely a genocide and there is some genocide that are worse than others