The 360 had a bit of a unique situation in Japan. “Arrogant” Sony put out a super expensive console and was pushing the Bluray next-gen narrative really hard. From my understanding they weren’t allowing visual novels, indie games, shoot em ups, or basically any games that didn’t showcase the PS3’s power onto the console. They only changed their stance after the poor sales of the PS3. The 360 was able to take advantage of this early on, but RROD and Sony changing their tune really put a damper on any momentum MS had.
Honestly, all it would really take for the Series to catch the 360 is one game that becomes a hit. A tall order to get a “Pokemon” but not impossible.
Also note that the Japan sales tracking we have access to is incomplete. They don’t track Amazon.JP and apparently the Xbox is doing well there.