It only needed four years to get there. Another example of how disappointing the PS4 has performed in Japan. When anything involving Wii U poses serious competition, it's safe to say that that isn't a good sign.
Nobody looks back at other gens top selling games and says "but it took years to get there". That was a pretty bad attempt at downplaying the monumental success I must say.
The fact splatoon sold over 2m makes this impressive. None of you even gave it a chance pre launch.