With the Japanese players, their sponsorship deals required them to play in league events in Japan. This was in addition to playing a full schedule on the WTA Tour. So, not only were they playing a lot of tennis, it was compacted into a short period of time. The players were simply being ground down. About 7 years ago, you couldn't go more than a dozen spots in the rankings without finding another Japanese player. Date, Sawamatsu, Sugiyama, Nagatsuka, Kamio, Endo, Hiraki, Kijimuta, etc. Several players achieved top 50 rankings at some point, and even top 30.
It may have changed in the last couple of years. Perhaps someone from Japan can elaborate?
As for the Chinese players, I wonder if the situation isn't similar. But instead of sponsors making demands on them, perhaps it's the government/federation that's pushing them, and perhaps causing them to overplay. They, too, may have to compete in leagues in China, or events in Asia, in addition to a full pro tour schedule. Perhaps someone from China can fill us in? (Where's Allison when we need her?