My guess is that it hasn't performed well enough to justify further development of free content. By September of this year, it had sold 1.35 million copies worldwide (that's three months after release), so it might slowly build on that, but it seems to be selling less quickly than the first Splatoon game did. Let's not forget that Miyamoto's gone on record stating that as a rule of thumb Nintendo tries to aim for a 2 million unit sales target on average with its games.
Also, let's not forget that Nintendo has to move their devs to making some new games. Even if the game is doing well, they have to weigh supporting it with further free content and development against taking their developers and working on something else new! You can't have your hitmakers not making new hits, after all!