Japan is notoriously forward thinking when it comes to technology applications. They may have freed up this space from legacy technology or maybe never used it at all as much of what they do has their own standards for technology.
Where as the rest of the world and the US is very slow to discontinue old services. So many bands around the world are tied up by old technology that depended on the upper echelons of the 2.4ghz frequency.
The link above comments on global military use, we may never truly know the real reason many bands are reserved other than for "Military Use".
Trust me there are many many frequency ranges reserved for government use.
