My understanding is Mandela was a respected leader who was willing to play ball and facilitate a peaceful transition where the white leadership got to keep all their property. That's why there's still massive economic inequality in SA today. Not to say Mandela wasn't admirable or that he didn't suffer, but it was a conscious choice to avoid outright military conflict at the cost of preserving an implicit racial hierarchy.
Take North Korea, say, another extremely nasty rogue nuclear-armed state. Even if there was a level of pressure that the international community could put on North Korea that would collapse it (it's already pretty far down the 'autarky' route), you can see that countries would be unwilling to go quite _that_ far, because there's no viable opposition and it would likely collapse in a very dangerous and ugly way.
Though to be fair none of these had happened prior to apartheid.