From Nintendo's POV there's just no reason to make MK9 for Switch when it would serve them much better as a system seller for Switch 2.
There are plenty of reasons why Mario Kart 9 makes sense on Switch:
1. Switch has not received its own Mario Kart yet and a prolonged lifespan for the system is easier to accomplish when you put system sellers on it in the second half of the lifecycle.
2. A new Mario Kart will drive profits for years, especially with the inevitable DLC.
3. The game can be easily repurposed for the Switch successor as a Deluxe version that includes all previously released DLC and some extra goodies. Most people don't buy DLCs, so Nintendo can put together an attractive package (read: system seller) even if the Switch successor is backwards compatible.
The reasoning that is usually put forward against Mario Kart 9 on Switch is pretty much the same as it is for a new 2D Mario game. "Oh, NSMBU Deluxe is selling so well, there's no need to make a new game for Switch."
It would be foolish of Nintendo to make the second half of Switch's life all about new IPs. They need proven IPs as tentpole games each year too. They are making Breath of the Wild 2, there will definitely be another Pokémon gen, another new 3D Mario game is likely too. I don't see why some IPs should be getting multiple new games on Switch while Mario Kart and SMB don't even get one, especially because Mario Kart and SMB are bigger IPs than the aforementioned.
There's no need to save up Mario Kart 9 for the Switch successor when the new console will receive new games anyway. Mario Kart 9 is bound to be a 30m+ seller on Switch and the hardware interest it creates will benefit all other software too.
If Nintendo decided against Mario Kart 9 on Switch, then we'd have a span of 10+ years without a new installment in Nintendo's biggest IP. It's already bad enough that Nintendo did this very thing to SMB (their biggest IP at the time, no new game for 16 years; Super Mario World in 1990, New Super Mario Bros. in 2006) and is about to do it again (New Super Mario Bros. U in 2012), so adding Mario Kart to this very shameful treatment of popular IPs belongs on the list of the dumbest things Nintendo could do over the course of the next several years.