All of the airports listed in the top 10 best airports are in developed countires, which show total usability of the airports are proportional to each country's economic power. (But, if Hong Kong is counted as a part of People's Republic of China, Hong Kong International Airport becomes the eception.)
On the other hand, world's cleanest airports are in the URL below.
All of the listed airports listed in the top 10 except for Zurich are in Asia. It seems that Asian airports are earnest in keeping themselves clean?
Rather than that, I think the fact that most of the Asian airports are recently renovated or newly constructed boosts their ranks up.