From what I understand, South Africa has historically been one of the more modern and economically stable nations on the African continent. Even during the Apartheid years in the 20th century, South Africa seems to be treated as a western nation up with the rest of Britain's former commonwealth states.
So being the most southern point on the African continent, South Africa has some legitimacy towards Antarctica as well. While a lot of Antarctic claims are due to power projection of globalist nations (US, Britain, France, Russia, Norway), other claims to the frozen continent come from nations very close by geographically, namely Australia, New Zealand, Chile, and Argentina. Even though some of these nations are regional powers at best, their proximity to Antarctica gives them a legitimate claim, from what I understand.
But South Africa seems to stand out, as being both in proximity to Antarctica and a regional power projection, and yet has no claimed Antarctic territory.
What was some of the historical reasons behind this?
Prior to the acquisition of foreign ministry powers by the Union of South Africa (1926, but definitively in 1931), the area of Queen Maud Land was generally seen as a Norwegian claim. It would have been difficult to lodge an additional claim upon, given that Britain was negotiating these matters around the time when South Africa developed its own external affairs ministry. The British Antarctic Territory itself only came into being in 1962, but its forerunner claims existed before the First World War, and the interwar era didn't produce any South African claim.
This isn't to say that South Africa couldn't have sought to horn in somehow, or that they weren't sounded out about it as the Norwegians formalized their claim and the British negotiated boundaries. However, as Lizé-Marie van der Watt and Sandra Swart have noted in their 2015 essay, South Africa really wasn't very interested in gaining administration over Antarctic territory in comparison to other British possessions during the flurry (ha ha) of claim-adjusting activity in the interwar period--and indeed South Africa had no actual policy about Antarctica. British efforts to gauge South African interest in 1929 as Norway was articulating its presence on the land to its south produced a response from Prime Minister Barry Hertzog's government that the Union had no objection to Norway's territorial claim, which the UK saw as a statement of disinterest. Not holding its own territory didn't prevent the planning of South African expeditions there, anyhow, and it does cost money to administer even empty territory. SA did seek to extend themselves south in the wake of the Second World War, to Prince Edward and Marion, with connections to whaling and regional maritime security instead of expansive aggrandizement on the southern mainland contra British treaties already settled. South Africa was, however, part of the negotiation of the Antarctic Treaty (1959-1961) and so is recognized as an active entity in the area despite having no mainland claims of its own. Today, aside from the islands and periodic personnel at other stations, South Africa maintains a station in Norwegian territory (SANAE) that is regularly occupied and supplied, and there are overwintering teams in those places.