Why did South Africa abandon the apartheid system precisely in the early 90s, and not earlier?

The apartheid system is characteristic of some African countries, where whites contrived to create their own dictatorships that oppressed the local population. This system was very similar to the racist order in the southern states of the United States. Any manifestations of discontent were brutally suppressed, demonstrations were shot.
South Africa managed to abandon the apartheid system precisely in the early 90s because in 1989 Frederick de Klerk came to power, who was a supporter of democratic transformations. He quickly abolished all the laws of apartheid, gave independence to Namibia, freed Mandela.