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 “Gravest Crime against Humanity” part honestly explains why so many countries abstained.

The slave trade was an absolute atrocity and certainly one of the gravest crimes against humanity but should we label it as the gravest crime? Do we really need to introduce a ranking between slavery, the holocaust and dozens of other genocides instead of agreeing that they are/were all bad without picking one as the worst?