This is presumably not only because of concerns over a possible erosion of deposits but also because the wrong perception has spread that Japan’s economic conditions worsened so much that such an extraordinary measure as the negative interest rate policy needed to be taken.
Financial institutions are facing the risk of a negative spread for marginal assets due to the extreme flattening of the yield curve and the drop in the yield on government bonds in short-  BIS central bankers’ speeches  5  to long-term zones into negative territory.