I think part of the reason why it hasn't so well so far is the fact that it looks super childish. I love the aesthetic, clay Kirby is super cute and the whole game is visually stunning, but I doubt that a 10 year old boy nowadays would want to be caught dead playing it. Let alone a 15 year old boy(I'm using males here because its still the primary gaming audience). I grew up with Kirby and still caught shit for liking the little pink puff ball. I always loved all the different powers and different ways you could tackle each enemy and level. I thought it was pretty neat, and to be honest, I thought Kirby was a bit of a badass. What other Nintendo character could cut you, stab you, light you on fire, encase you in ice, stick you in a whirlwind, electrocute you, then just blow your ass up all while dodging apples?
I also think that a huge part of it was the gameplay choice. If there's one thing people generally don't like, its the loss of control of their character. Sure you guide Kirby with lines, but you don't actually move him. Not to mention that Kirby is know for the different powers, simple platforming, interesting level design, and fun gameplay. Some of that exists in Rainbow Curse, but the focus on a variety of powers is absent, the level design is decent, and the gameplay isn't very fun when you have an HD Kirby game you have to play on the gamepad exclusively.
I honestly think that Kirby is kind of a perfect storm of what gamer's just don't want anymore. It makes me sad to admit it, but its true. I think it might have sold better if it were a more traditional Kirby game. Even then, the little pink power puff needs a bit of an overhaul. Maybe a 3D environment instead of the traditional 2D. Something similar to Starfox Adventures(Yes, that game is awesome even tho it isn't traditional space combat).
“What I say is, a town isn't a town without a bookstore. It may call itself a town, but unless it's got a bookstore it knows it's not fooling a soul.” - Neil Gaiman
