system
Given the following post, respond as if you're commenting on Reddit:
human
[The Man in the High Castle] Why is the I Ching such a big thing in Japan-ruled America?

The I Ching is a Chinese work, not a Japanese work, and as far as I can tell, it never got hugely big in Japan, not even during Imperial Japan. Also, I'm pretty sure it was the Sino-Japanese War that helped kick off WWII, so I don't think Japanese people would want to promote a Chinese work.
gpt
Chinese culture has a vast influence on Japanese culture. Kanji is derived from Chinese writings. Shinto and Zen Buddhism was brought in by Chinese monks. 

I-Ching would have been co-opted by the Japanese and "re-packaged" as a Japanese original work.