One reason for this is the increasing popularity of these marks. In fact, the JPO notes that Japanese companies have been filing for and receiving nontraditional marks in other jurisdictions.
A draft version of a bill to amend the Japanese Trademark Act looks to provide for protection of color marks, sounds, moving marks, holograms, and positional marks. However, the draft bill does not provide for protection of scent and taste marks, due to concerns that it is too difficult to specify the scope of such marks and there is doubt that there is immediate need for them.