In addition to communist countries, South Africa was a noteworthy abstainer from the vote for the UDHR. That's because, in 1948, South Africa's government was mostly based on violating human rights.

In 1948, South Africa's National Party established a government based on a system of apartheid. Apartheid is essentially legal racism, only worse. Under this system, segregation between the races was not only allowed—it was actually required by law. Kind of like Jim Crow laws but applied on a federal level.

The government of South Africa kept a registry of people based on race and used this information to enforce segregated housing. Black South Africans, the majority of the population, were often forced to move outside of city centers where the white ruling class lived (source). Other laws prohibited interracial marriage (definitely a violation of the UDHR's Article 16) and prevented people of color from enrolling in universities (source).

Apartheid in South Africa didn't really end until the 1990s, when leaders of the African National Congress managed to negotiate a peaceful dismantling of the system. In 1994, South Africa established its own Bill of Rights, which pretty closely resembles the UDHR.