I take Xbox One's low numbers as Microsoft giving up all their ambitions in the Japanese market that gen, they basically must have stopped even shipping units to the country for most of that generation. With Xbox series they probably pushed hard at the beginning with the hope gamepass would give them a decent market share in the country, but when Xbox series sales quickly lost steam, they once again gave up in Japan and shifted their shipments fully to western markets which led to the current 300 weekly sales situation in the country to happen once again.

One thing i think is a problem for Xbox series in Japan is that a market that is still buying so many physical console games will be hard to shift to a digital only gamepass machine on a short notice. Much of Japanese console gaming is still about old school buying games at large electronics retailers or game stores, where Xbox is insignificant.

Some of those Xbox Series sales (just like PS5) came from scalpers reselling overseas. Now even with that I still think at least 400k remained in Japan. I think what made the Xbox Series appealing are 2 things:

1: Japanese publishers were now releasing everything on PS and Xbox, so a lot of franchises that used to be PS only are now on Xbox. The biggest games on PS in Japan are 3rd Party, not 1st Party as the only 1st Party PS game that even gained a lot of traction in the PS4 generation was Ghost of Tsushima. 

2: I think backwards compatibility played a role as well where a lot of older games cannot be played on a modern PS console, but Xbox has them, such as FF13 Trilogy, Nier, Star Ocean Last Hope, etc. 

I find it quite interesting that Xbox improved over the Xbox One in Japan, yet PlayStation is the opposite where it looks like the PS5 will sell less than the PS4. Nintendo and PC honestly just took over.

Imports. Xbox is not released at retail in many markets, so people buy it in Japan and import to their countries