South Africa was not a particularly willing participant in either World War. Since the mid 19th century, and especially after the conclusion of the 2nd Boer War, there was a great deal of animosity towards Britain and the British in South AFrica. In the First World War, a declaration of war triggered a failed coup by Afrikaner Nationalists.(Berger, Iris. South Africa in World History, 2009). The failed coup was violently put down but the anger and hate towards Britain was not. It is really important to recognize that Anglo-scepticism is a major theme in the history of South Africa.

By World War 2 Herzog's National Party was the major force in opposition to Smut's United Party. Smuts who was a liberal in the British terms of the times was up against Herzog who's party supported the Nazis. Robert Citino book "Germany and the Union of South Africa in the Nazi Period" details the relationship between Herzog's party and Nazi Germany.

Smuts party would stay in power, but by 1948, the National Party, now led by DF Malan and Hendrik Voerwoerd would come to power, and thus begins the Apartheid era.

Remeber anglo-scepticism? Well Apartheid was as much about seperating black and white as it was about seperating British and Afrikaans(Erich Louw. The Rise, Fall, and Legacy of Apartheid, 2004). As such the National Party would have much rather played up heroes of the Boer War and build monuments to Voertrek, than to honor a "british" war. This is despite the fact that between 50-70% of volunteers in 1939 were poor/low class afrikaans speaker(although the litereature does not clarify if this means Boer or british descendent speakers)(Neil Roos, Ordinary Springboks, White Serviceman and Social Justice In South Africa, 1939-1961). I will mention that this does in certain ways play into the national parties rhetoric about the poor afrikaaner which became a major part of their campaign(basically the british industrialization efforts have come at the cost of the rural afrikaaner farmers.)

So really South Africa's contribution gets downplayed because of the naimosity between Britain and South Africa. The fact that the post war government was at the very least moderately supportive of the Nazis, and had little interest in honouring the British aspects of South African history outside of the colonial relationship, you get a situation where things like this are downplayed.