It’s not just Japan. Country music is also popular in England. I took a ferry ride from England to Gothenburg, Sweden in 1992, just so I could say I’d been there.
I was only on the ground for 6 hours. I and a friend walked to a mall near the ferry terminal, and we saw American light trucks everywhere. While walking thru the mall, we saw and heard an American playing guitar and singing. You guessed it, he sang country music.