Well, let's start with decades of apartheid which enforced racial segregation, curtailed the rights of blacks, and maintained white supremacy, creating deep, and still lingering imbalances in wealth that made the country one of the most unequal in the world.
And for that reason... as the program has long and continues to be broadcast on South Africa’s M-Net network (via DStv), a subscription-based private satellite channel, the subscriber base has largely been white, because blacks in the country haven't been able afford the necessary subscription fees (although that's gradually changing, as more black people are able to afford luxuries like satellite TV); and since winners are selected by votes from audiences, with a predominantly white subscription base, it should be maybe no surprise that the winners selected each of the last 7 seasons have been predominantly white (there has been 1 mixed-race winner).
