Because single players are more capable, and more competitive. As a single player, you have to take care of the whole court, while the doubles, just to protect the half. Single has to develop your own strategy, while double just follow the shuttlecock quickly. So when you play single, you have to use you brain, while for double, you need your eyes. So when you play single, you are like the commander at battlefield, while for double, you have to be a diligent soldier. A lot of top double players are converted from 2nd class single players with no future in single. Such as Cai Yun, Sun Yang, Zheng Bo, Fu H...... Sun Jun claimed several national double titles after retired from single, now Xia XZ will play doubles too. Koreans play doubles early, so that's the trick they succeed. That's good, but most people want to be the hero himself, it is very difficult to convince them to play doubles. Koreans like to obey orders, I guess.