Well thats a decision nintendo has made. Maybe they thought people that care about achievements are not likely to buy nintendo systems and games anyway
Well they choose not to because frankly people that care about achievements most likely moved on to the competition and nothing nintendo will do on that front will bring them back unless its something truly groundbreaking in the achievements department
Well I'm not sure about groundbreaking, but they were in the perfect position to introduce a meaningful achievement system. Microsoft and Sony laid the groundwork, but really only did the bare minimum.