Seems to be mainly because Bluesky is just a better user experience.

Changes in terms of service regarding AI (artificial intelligence), changes in the block feature and the presence of so many bot-like ‘impression zombie’ accounts are viewed negatively by Japanese users

But, X is still incredibly important in Japan and Bluesky isnt a serious threat to it culturally right now.

X is still a major, major news source for Japanese users, so it really won’t slow down anytime soon. Newer platforms like Threads or Bluesky really don’t have any meaningful adoption or use for brand marketing in Japan — yet