He explains why on his site: ( http://taichi.graphics/misc/ ) and in another comment ( https://news.ycombinator.com/item?id=13327362 )
Then why “太極” instead of “太极”?
“太極” is a word from not only traditional Chinese, but also modern Japanese. Considering the fact that a significant part of Taichi (10+ papers and the general software framework) was done during my internship at The University of Tokyo hosted by Prof. Seiichi Koshizuka and Prof. Toshiya Hachisuka, I thereby chose this word in the intersection of two languages in memory of my time at UTokyo.