There’s normally a reason when that assortment of countries chooses to abstain (the no voters are normally just evil). In this case it’s likely the use of the word “gravest”. I’d say the holocaust was worse, at least in the slave trade the people were just a means to an end. The holocaust involved torture by design and aimed to erase an entire religion.

Others may disagree, but there’s at least room for doubt on the declaration that it’s the “gravest”.