Mostly tradition. Tennis got a great tradition in the slam countries, like US, France, UK and Australia
In South America and especially Asia the tradition isn't as strong. South America has been coming strong with mainly Argentina but in the history of tennis still fairly new.
Asia has been lacking a strong tennis tradition and only had sporadic good players. I think the reason they place tournaments in Thailand, China, Japan and so on is mainly MONEY. It's a big fan base for TV and merchandise which translates too $$$$
China after all has a masters in Shanghai and that is a big step forward. South America should focus on getting a Masters. Would make sense to perhaps have one in Brazil or Argentina
Glad that you realized your question was xenophobic in leaving out Africa and Antarctica.
So I'm guessing that you'd like to see 10+ Grand Slams annually? I'm all for a cold weather outdoor tournament, btw, as I've always thought tennis strongly favoured players who could adapt to heat better.
Is this joke thread of what? because those slams are tradition , history they are were they are for decades- with AUS switching, but still the same country, and when they started slams in late 19th century, they didn´t know what was tennis in Asia or South America in those times, it was sport - white sport for high society on grass.
South Africa does have a long history with tennis. Argentina or Brazil has also had good players and tennis is popular. Really the biggest problem is there are only 4 slams so you can't just put one in every continent. Personally the FO should be moved to South America, after Nadull winning it all the time there's really no history left and no reason to keep it in France.