Since the first games, many fans have clamoured for "Assassin's Creed in Japan"... I suspect they fantasised about being a male Japanese ninja running across rooftops in the moonlight, sneaking over nightinggale floors, and silently assassinating people in their beds, combining what they saw in Altaïr and Ezio, with every Japanese media trope they knew of.
But in the intervening years, Ubisoft and/or the entire AAA games industry is now seemingly driven by a need to conspicuously showcase diversity and inclusion (from some gamers' viewpoints). You could also view it as the gaming industry trying to broaden its audience and get out of the pigeonhole of catering to the base desires of sweaty manchildren, but either way, it's upsetting a certain type of game consumer.
So, when Ubisoft finally got around to setting Assassin's Creed in Japan, and they picked pretty much the only person around that time period who wasn't Japanese as the main protagonist, seemingly to meet diversity goals, capital-G Gamers went bananas over it, like it was a personal affront to them.
* There aren't that many real historical accounts about him so people can argue all day about stuff like "Was Yasuke a real samurai?" without clear evidence about who's right.