I would argue that, more than six decades after the institutional framework for a parliamentary cabinet system was snapped together under a U.S. military dictatorship, Japan has failed to institutionalize cabinet government. The greatest deficiency lies in factors that inhibit the cabinet's ability to impart tactical direction to policy. In fact, the cabinet has never become the foremost executive organ as expected in a "Westminster system," and this has contributed to its mixed record in responding to critical challenges.
The autonomy of cabinets has never been assured, as inherently weak prime ministerial leadership, government officials who exercise considerable power (e.g., policy agenda-setting at the vice-ministers'
gatherings that preceded cabinet meetings), a ruling party that ruled almost perpetually from 1955 on (enabling it to institutionalize an intra-party system of pre-clearing all major policy and budgetary proposals before cabinet consideration), self-governing "policy tribes," and the uncertainties of coalition governments and Twisted Diets kept the cabinet from assuming its prescribed role. Although, on the surface, Japan's cabinet system resembles the Westminster model, in practice it does not.

There is much, much more to it than that. I explore this in depth in a forthcoming book.

Brian