I'm not really convinced Wii U got a bump from Mario Kart that lasted more than a week. It sold 140,000 in five weeks, not four for June, and 83,000 in July for four weeks, not five, so you're looking at 28,000 / week in June to 21,000 / week in July. June is when people graduate from school or college often times, and it's Father's Day, it's typically a bit better than July for everybody on a weekly basis, so it's hard to say if Mario Kart really did anything in the US.
I've always worked on the assumption that the original Wii sold because it had a monopoloy on motion control gaming from 2006-2010 and then declined when it no longer had that monopoly. Every game in that 2006-2010 that expanded the monopoly into new genres reinforced the Wii or prevented it from weakning - the kings being Mario Kart for racing, Just Dance for dancing, and so on, but now that there is no motion monopoly, games are just games, and the people buying them are people who liked Nintendo, not people who liked Nintendo or liked motion gaming.