A bit strange that Japan has never got heavy coverage since, y'know, Japanophilism is one of the big backdrops to cyberpunk.
Shadowrun wrote itself into a corner early on, ad it took quite some time to work itself out of it.
In the early fiction and fluff, the Japanacorps were everywhere, and involved with everything. Half the original "Big" AAA Megas were Japananese, and they had a major influence on the world (reflecting the late 80's belief that Japanese business dominance would continue). Hence, Nuyen. And there was a lot of little Japanese phrases and slang that were commonplace in the fiction as well.
But Japan itself was written as this hardcore, imperialistic, completely racist (to both gaijin and especially any metahuman), and totally corporate dominated. There wasn't any ROOM to actually play a game set in Shadowrun shy of playing a team that was completely human and completely loyal to one corp and worked exclusively for them.
Over the years, and especially with Year of the Comet, we tried to shake that up and loosen it up a bit. A couple corps moved away from Japan (or fell apart, like Fuchi), and a new Emperor with a different outlook rose to power. This loosened the corporate stranglehold a little and relaxed the racism a bit. It's still not the best place to run though.