Mario Kart 8 numbers are no way impressive. It should be expected. Thinking otherwise is underestimating Nintendo and the quality and popularity of the Mario Kart games. They have all sold extremely well and sell better than Zelda and Metroid titles. If it didn't help sell Wii U units itself than Super Smash Bros would be the last hope for a major surge.
Watch Dogs is more impressive simply because its a new IP so it has no history other than the hype behind it.