I know sometimes Nintendo appears to think it's big franchises can sell themselves so unless they are planning to make Splatoon a big franchise I don't expect it to get much advertising love.
I think people don't realise how much money can be wasted on advertising with no guaranteed return, so in effect in can sometimes be like throwing good money after bad. I think lack of advertising by Nintendo these days is very much a symptom of them deciding that they want to get out of this generation with the simple goal of minimising losses/returning to profit.