Because they had a very much blank slate after the war, when the old order was not only toppled, but ashamed and humiliated, and the old good-old-boys networks were destroyed or at least weakened.

Furthermore, the nation was hungry, not literally, but hungry for success, so they worked extra hard. The success of post-war Japan is built on the 16-hour workdays of countless salarymen. Those peculiar post-war conditions are long gone.