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?