>>713999716
I can't recall where I read it at this point but basically, since the story of the manga was an adaptation of "The Tournament at Sunpu Castle", it was supposed to continue with going into the story of all the fighters in the manga. Including spearbro, pacifist swordsman, the frog and the tomboy. In the book it eventually ends with Fujiki and co trying to quit with Tadanaga's insane tournament, and we know things did not end well for Tadanaga historically either.
This meant you'd get some in-depth "backstory" for all the fighters, probably being just as haunting as Seigen and Fujiki's, but the issue for Yamaguchi as far as I heard was that it was legitimately making him depressed to keep working on it.
I think ever since then his manga have been much happier in tone as a result.