Have you played the game recently? The reason why people get so into the universe is really evident even at first glance.
And by the way it's not only a Japan exclusive, Splatoon 2 has sold more than BotW in Spain for the past 5 months for example, and it's currently doing the same in Italy too. Globally speaking it sold as much as Odyssey in the last 3 month report. 760k Odyssey, 740k Splatoon 2.