If memory serves, they were already an older team in a sport that's undergoing a youth revolution right now. They got older.

Most national teams struggle to maintain success across successive tournaments because four years is a long time in soccer (most major tournaments--Olympics, (W)WC, Euros, etc. are 4 years apart). A group in its prime in a given tournament will be old by the next. National associations have to do a huge amount of development work to keep the pipeline going, but national associations rarely have the resources to do this--it falls to the clubs. Clubs in women's soccer are growing but still hit or miss, and options in Asia aren't great. I wouldn't be surprised if a lot of the Japanese players struggle to find consistent playing time, especially as their careers wind down. Contrast France and Germany, both with some excellent young talent and great domestic women's leagues where said talent can actually play.