I have noted this as well. I think it boils down to four things. One, the education system is getting better at teaching English, perhaps because, in the case of China, students get an earilier start, teachers are better, or parents as a group are better able to help their children at home.

Second, students get to hear more of the target language they are learning outside class. They get more chances to practice the language which helps move them out of the Elementary stage.

Three, students as a whole are moving ahead faster so that the performance bar has risen and only students who have a chance of reaching near fluency really want to put the effort in. Students who get stuck at an Elementary level give up. Some students who learn do so to compete with others.

Fourth, the population of a country has stopped growing and their are fewer young people adding numbers to the “I want to learn a language pool” (this describes Japan, for example).