Because it's basicly a portable, and Japan mostly buy portable; it's a "life style", and a different culture. It doesn't matter Switch has no new games, they would buy it anyway.
What surprise me is people who still get surprised about it :D
Just like when vita sold more than ps4, right?
A portable from Nintendo sells better than Home Consoles in Japan, regerdless of new games; PS4 is feeding their gamers much more week by week since Jan, but Switch keep selling 3 times better with no new games = A PORTABLE from Nintendo sells better regardless of games. I think it's cultural, and life style, and also Switch looks like a very sexy and colourful hardware to buy, like a new trend. Sure it's not the deep and crowded game experience.
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