Unlike other abominations like racism, sexism and apartheid, caste system is not challenged by international forums. Because it is strongly fused with Hinduism. And Hinduism with it's good looking outfits - like vegetarianism, Gandhi and non-violence, yoga etc. - have a international acceptance. Another reason is that the caste system is not colour coded like racism, so it is hard to get noticed, at-least from the outside. Also unlike apartheid, the caste system has admirers in high places.