If you think the sales disparity between the Xbox One and PlayStation 4 is drastic in the US, multply the gap by 4x and you'll have the difference in number of consoles sold in Japan. According to former Chief Xbox Officer Robbie Bach, it all goes back to the first Xbox and the fact that Microsoft should have simply just ignored Japan and focuses on territories where Microsoft had a stronger foothold. Bach reflected, "It’s just a question of focus. And it’s a question of where you have assets and where you don’t have assets. And we had no assets in Japan. [Microsfot] didn’t have relationships with Japanese game developers [excerpt] for a few isolated ones [for games like Ninja Gaiden], which were actually more popular in the West than they were in Japan."
Bach stated that the effort, time and money spent on trying to gain a foothold in Japan kept those resources from being allocated to things where the Xbox actually had a chance to make progress.
In the video below, Phil Spencer (Head of Xbox-current), Peter Moore (Head of Xbox - 2003-2007) and Seamus Blackley (Xbox Creator) echo Bach's statements and discuss their frequent trips to Japan to try to get Xbox off the ground.