The typical explanation given is poor economic policy - good times led banks to start lending excessively, excessive lending drove speculation, speculation drove bubbles, those bubbles burst, everything collapsed, and they just never managed to get back up again. But that last part is what most interests me.
Most of the Soviet Bloc type nations you named had reasonable fertility rates under the USSR, only collapsed once the USSR collapsed in 1991, and are inching back up again. Japan, by contrast, never had a particularly great fertility rate post-war. Then it just collapsed in the 1970s and simply kept going down. [1]
Fertility rates just determine so much. With a good fertility rate companies grow naturally because their market size is constantly naturally increasing. And vice versa for poor fertility rates. It also determines the overall age distribution of a population, such as how in Japan now 10% of the population is over 80. When an ever larger chunk of your population is leaving the labor force with not enough people to replace them, and then an even smaller taxpayer base to provide the social benefits necessary to sustain these people in their elder years - everything just becomes completely unsustainable.
And, again, none of this will ever get better such as, for instance, when the older people start passing en masse. This is because fertility rate determines the ratio of distributions within a population. You can see this quite clearly in this [2] population pyramid. The only solution is for them to suddenly start dramatically increasing their fertility rate.
[1] - https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?location...
[2] - https://en.wikipedia.org/wiki/Demographics_of_Japan#/media/F...