First, there is confidence in the market that Japan has the will and ability to eventually address fiscal consolidation. The second reason is that investors anticipate that yields have been stable thus far and will continue to be so.
It may be hard to believe at this juncture, but in the euro area, every country issued bonds at the same level as Germany up until a few years ago. Of these two reasons, I believe that the first one is more convincing as an explanation of why a severe fiscal situation does not lead to higher bond yields in the market.