Well, those treaties are not particularly effective in the first place, because they don't include any automatic enforcement mechanisms or penalties for not complying with them.
International conventions like these are really more statements of intent rather than enforceable contracts. Ratifying them basically means saying "We agree that apartheit/genocide is bad, and we pledge to persecute and punish people who do it". But that's really all they are: a pledge. As such, they are as easy to back out of as they are to enter.
Now you might ask "Why not give these conventions some teeth? Can't the UN somehow force the contracting parties to comply?". The answer is no.
- The UN is a rather weak institution. It avoids infringing the sovereignty of its members. And when it takes measures to enforce its agenda through sanctions or interventions, then it only has the resources available which member-states are willing to spare. This is by design! Because:
- States are generally reluctant to give up sovereignty. Treaties which would impose too many restrictions or demands on states, infringe on their ability to legislate and govern the way they see fit or would be impossible to denounce, would be treaties very few states would be willing to sign. So when you want a treaty to get ratified by the majority of UN member-states, you need to make it weak and vague enough that it is palatable to them.
Which is why "treaties with teeth" are usually bilateral treaties between two states or small groups of states, and not "big-tent" international treaties.