As living standards in third world countries rise, the odds start to grow in their favor when it comes to excelling in any industry or sport. All other things being equal, you're simply more likely to find dominant athletes among a billion people in China than among 65 million in Great Britain.
True. I think that also holds true economically as well — in the 1950s, the USA was the only real economic power around: Western europe and japan were devastated, Russia and its satellites were also devasted and under Stalinism, the rest of the world was recovering from colonialism