Yeah! Especially for a New IP! 

Especially for a "W101" Type of Sales :p

Three major things that helped.

1.) Character design, I mean my goodness whoever crafted the inklings did a superb job. Despite what many people think this isn't the first original IP Nintendo dropped since Pikmin, but in terms of marketabliity this is the most polished and simple enough that even with minor changes (via in game equipment or just some of the extremely well done fanart) it allows for creativity while being distincive in being a inkling.

2.) No competition. Smash for Wii U was competing against Smash for 3DS, Nintendo may have said otherwise but I think we all know that is the case. And given the Wii U's weaker install base, it isn't any wonder that it did poorer in Japan then Splatoon, where there is only one area you can get Splatoon. If there was a 3DS version for Splatoon it would have destroyed the Wii U sales because that is the dominant video game device in Japan.

3.) Advertising. Nintendo was pretty smart to put up Mario Kart level of advertisments for this game from the stupid songs,to other stuff. It got people talking, the more they talked the more some got curious and found out it was a great game. I myself wrote this game off but Nintendo did well in getting this game out there and supporting it which has made it one of their better success stories. 

Pretty much this; you summarized pretty well why Splatoon was so successful. I'd also add that TPS was a genre almost missing on Wii U.

Haha, this is delicious. I remember when people were saying it will be lucky to sell this LFT WW

So Wii U should be having its next million seller in Japan before the end of the year! Though that shouldn't be the only reason we should party! But also because this would be the first Exclusive Gen 8 Home Console Game to pass 1 million sales in its release year(calendar year, not in the 52 week span)in Japan!

and maybe 1st game to do so without bundling. 905k=Sales, so including shipped should be around 950k+ and be over 1mil about the same time as the bundle comes out imo

Yep, actually most probably as this is typed that Splatoon has already passed the 1 million shipped mark in Japan, because going by sales to consumers, parting from the week before the end of September as said by the link given till the last Media Create numbers we have.  When it comes to retail sales it has been 67k, so adding that to the 905k number, it would sit at 972k, then we add around 10-18k from digital, which would sit the game at 982k-990k, and there should be more than 10k copies outhere in Japan, so shipped numbers should alredy be over 1 million as sold numbers will get there with this upcoming Media Create update.