Because if they don't make Mario Kart 9 on Switch, then there would be a 6+ year gap between Mario Kart releases, and 9+ years between brand new versions. This would be utterly ridiculous considering that Mario Kart is Nintendo's most popular franchise as of right now. It's the top game on both 3DS and Wii U, and it's number 2 on Switch despite just being a port of the Wii U version. We haven't gone three years without a Mario Kart game being released since 2002.
Here is where you wrong, because you look how Nintendo managed MK games on two different platforms in same time, but huge difference is that Switch is becoming only active Nintendo platform, and every Nintendo platform had only one MK game. So it big question if Switch will receive another MK game at all, and if it gets, when exactly could get it. IMO if Nintendo is releasing another MK game for Switch I dont see it before 2020. because MK8D will keep selling great.