Not a personal fan of Yakuza; but I recently saw some footage of Yakuza Ishin and it sparkled my interest; don't now why sega dosen't release it outside japan given the fact that Ghost of Tsushima was a major hit for sony; I think they are really missing out on this opportunity.
They didnt because by the time Kenzan and Ishin came out, (before Yakuza 0)
the Yakuza franchise was not very popular in the west. It was a series for hardcore Japan fans.
It took a lot of time to convince Sega to translate all of the main series games
and some of the spin off games they released in english, were even less popular.
So i guess they probably thought that a game like Kenzan or Ishin could only fail in the west
because the setting and story is even more niche.
You have to remember that this whole Japan hype - travel live streaming from Japan,
old Japan setting hype (Nioh, Ghost of Tsushima, Sekiro), newer releases like Persona 5 or Yakuza 0,
is pretty new. Big Youtube channels like Abroad in Japan or The Anime Man are only 6-7 years old.
This whole worldwide Japan push happened in the last 5 years
but Kenzan and Ishin were not released in that time frame.
Segas new attempt started with Yakuza 6 and Yakuza 0
and then the translation team always had to work on the same game the japanese dev team was working on
because otherwise they couldnt keep up with them.
So they had to work on 6, 0, Kiwami 1, Kiwami 2, 3-5 Remaster, Judgment Fist of the North Star and Yakuza 7,
all of this in 5 years. There was simply no time to work on a english version of Kenzan or Ishin
and Sega could not foresee that there will be a feudal Japan setting hype.