Japan has (and is) suffering an extraordinarily low birthrate, while its historical background suggests that its citizens are raised on a Confucian tradition, which lauds the bearing of children.
Moreover, how come the polices the government has implemented haven't helped?
I don't have the sources available to go into any real detail, but Japan actually has a history of limiting the birth rate. Productivity gains in Japanese agriculture in the 18th and 19th centuries did not lead to the rapid population growth that is often seen in pre-modern societies.
The cultural factors at work seem obscure, perhaps because birth control was not a subject for polite and official discourse. There's some evidence of peasants maximising income. The tradition of primogeniture rather than partible inheritance may have had an impact, and perhaps also the relative stability of Japanese society. Japanese peasants must have practised birth control in some form, and there are examples of infanticide being practised.
(Sources: Hanley, S.B., Yamamura, K., Economic and Demographic Change in Preindustrial Japan; Irokawa, D., The Culture of the Meiji Period)