Japan was able to keep interest rates at zero for 15 or more years because

the economy was in such bad shape that there was no demand for funds and so lenders had to take low returns.
the Bank of Japan did not issue enough bonds to fully drain the reserves added by the large daily fiscal deficits.
the Bank of Japan were convinced that inflation was under control.

Answer: the Bank of Japan did not issue enough bonds to fully drain the reserves added by the large daily fiscal deficits.

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