Mario Kart 8 was a pretty cool racing game for the Wii-U when it was released back in May 2014. Mario Kart 8 has managed to sell over 7 million units worldwide on the Nintendo Wii-I since then and was also re-released for the Nintendo Switch as Mario Kart 8 Deluxe. The Nintendo Switch version of Mario Kart 8 has managed to sell over 1.7 million units worldwide since its release back in April 2017. As we all now the prequel to Mario Kart 8 known as Mario Kart 7 was initially released for the Nintendo 3DS back in December 2011. Since its release for the Nintendo 3DS the Mario Kart 7 has managed to sell over 13 million units worldwide which is significantly higher than its successor Mario Kart 8 for the Wii-U.
Since its release back in the Spring of 2014 Mario Kart 8 has only managed to sell over 7.4 million units worldwide. If Mario Kart 8 was released as a 3DS game it probably would not have the amount of success as it did. In fact, there may not have been a Mario Kart 8 Deluxe if MK8 was released exclusively for the Nintendo Wii-U.