The US did not abolish slavery from their own accord. It is another funny thing; As popularized by liberal societies, an opinion to an issue can be represented by a five position thing, no opinion, favourable, favourable but, disfavourable but and disfavourable.
While both sides during the civil war shared a belief in the human rights and while the main cause of the war was slavery, no side went to war to end slavery. The confederation was favourable to slavery and the Union was favourable but to slavery. A big achievement. Both sides fought a war to perpetuate slavery while claiming they were for freedom. It is very funny.
Slavery was abolished in the US against the will of WeThePeople, just as the first generation of US americans perceived it. When reading them, it appears they considered slavery would end in the US when the US would lose the capacity to coerce the negro so they stay on the plantation. From the very start, a lot of time and power was allocated to maintain the negro on the plantation, in fact, from the independence to the civil war, the US increased the strength of the slavery institution. It must be reminded that people during the war of independence were so obsessed with maintaining the negro on the plantation that they preferred to patrol plantations rather than going to fight the English.
What happened during the civil war is that the confederation lost their capacity to coerce negroes to stay on the plantation. Negroes simply left as the requirements to coerce them were no longer met.
From this point, it was understood that slavery had to end in the US, not because of liberal tenets but because the capacity to coerce negroes was gone.
As to South Africa, already stated it, the Apartheid has to end because Kolmanskop is coming to South Africa and having a white people government in charge when it happens would be too telling. Not diminishing the struggle put by people who fought against the Apartheid but a main cause of their success is that Kolmanskop is coming to South Africa, which is heavy and not many people live in Kolmanskop and a huge decrease in population is likely to happen in South Africa to match this situation. Better not to be in charge.