Back then, Formula One cars were pretty abundant. You could buy a year or two old chassis that is floating around the world for fairly cheap, get the Cosworth motor and boom! A race car. Get a few together and boom! A racing series. Essentially historic series. No big names have come out of them (including the Auroa F1 series in the UK).
Plus even only 20-30 years ago television wasn't broadcasting the Formula One around the world like now. You start these series and help promote the sport so when the Grand Prix comes to your country (which strangely coincidental Argentina and South Africa no longer have a GP), people have a good idea of what they will be seeing. With the advent of world broadcasting feeds and the internet, sports in general are world wide. You don't have to wait until your countries Grand Prix to see F1. You tune in 20 weekends a year and you have F1.
There was the push in the 70's and 80's for having American football leagues around the world. And while early appeal, fans only wanted to see the NFL. So broadcasters and companies switched from promoting a separate league to developing a system to broadcast the NFL world-wide.
America is an oddity. They developed their own series aside from F1, so when the Grand Prix came to town, people where just not as interested. They had USAC, NASCAR. F1 to Americans even in the early years was just nothing different.
I think the racing scene is South Africa died when the colony roots started to disappear. Which for the country was a good thing, best thing to happen to them, but it also changed priorities. It was a push in sports to what appealed to the masses. And the white's of colonist background no longer had nearly absolute power. They no longer had all the money. It was a culture change.
As for Argentina, I couldn't tell you. The nations motorsports scene is pretty strong, but abroad the country just isn't. Some of it goes down to money. There have been some good drivers in the last couple of decades, but due to financial reasons they haven't succeeded.
But the same can be said about Brazil. Columbia and Venezuela have become the go-to countries when looking for rising talent in South America. Brazil used to have upwards of 10-15 kids running the European ladder series'. When they struggled, they came to the U.S. and soon where winning races and championships in the U.S.
Motosport goes through cycles. Sometimes racing is strong in some nations, and weak in others. In the 80's and 90's Canada was pumping out drivers like crazy. Great drivers. Paul Tracy, Greg Moore, Stephan Proulx, John Jones, Jacques Villeneuve. America in the late 50's, early 60's competed with Britain in producing the world's best drivers.
But I went off topic a bit, I think. But yeah, cultural changes occurred (maybe due to changing financial states), and the people of the nation moved there money and support elsewhere.