It's because of China's approach to sports. The government invests in sports as a form of "sports-washing," but it never does it at a grassroots level. It simply picks sports that aren't as popular to invest in because that's the easiest path to success. Look at the last summer Olympics. China won 38 gold medals, but of those 7 were in weightlifting, 7 in diving, and four each in table tennis and shooting. That's 22 all together in sports that I'm not sure exist at a professional level around the world. I mean, we've all played table tennis at some point(there was a table in the rec room of my frat house), but I'm not aware of existence of any kind of competitive leagues in North America.
It's actually fairly cheap for China to invest in these kinds of sports, and the money is more likely to go a longer way. Plus, you only need to produce a couple of phenoms and you're good. Team sports are very different. You need grassroots investment and broad popularity of the sport. You need a large pool of potential pros so that eventually, you can send the best to international competitions. You need a youth system of some kind(academies, or like in States, colleges and high schools) to produce talent and to separate the wheat from the chaff.
And the giant centers are an obvious corollary of this. There is a very small pool of 7-footers between the ages of 18 and 35 in the world, so it's easier for countries without a strong basketball culture to produce those than it is to produce world class 6-footers. It also has to do with skillsets being more complex and needing a different environment to develop. Like in (association) football where countries with weaker systems will produce forwards and strikers who may be world class, but producing a world class defender or goalkeeper is a whole other thing.
There's a reason why so many early Euros(and Aussies, actually) in the NBA were bigs and why even today, there's an imbalance among international players in the NBA as far as size goes. And the ones who are guards tend to be from countries who have strong basketball programs. There have been 6 players from Republic of Georgia in the NBA and she shortest one was 6'9. Three Poles and the shortest was 6'11. Meanwhile the French, the Slovenes, the Germans, the Serbs, the Canadians and the like come in all sizes because those countries basketball programs are elite. As you can see at the current FIBA world cup.
