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.[0]: http://taichi.graphics/about/
From my reading, traditional characters are still in use in China (distinguished for the purposes of this discussion from Taiwan).
Traditional characters are used informally in regions in China primarily in handwriting and also used for inscriptions and religious text. They are often retained in logos or graphics to evoke yesteryear.
https://en.wikipedia.org/wiki/Traditional_Chinese_characters...
In trademarks and business signs, TC is common.
https://www.quora.com/How-do-people-in-mainland-China-feel-w...
An application name is not all that different from a business name or a logo. Given this, choosing to use traditional Chinese characters (in this case 太極 in contrast to the simplified 太极) is consistent and not necessarily unusual. Of course, this is all speculation without hearing from the author.
1. There seem to be more phono-semantic compound characters.