What makes no sense is leaving tons of money on the table that would also please a majority of the fanbase. I don't think Nintendo is going to ignore the fact that far more fans like Mario Kart than any other racer just cause a port already exist on the system. A 20+ million seller like MK9, even if it were to cut MK8D's legs to 0 (lol no), would still net a lot more profit than the 2-3 million sales Diddy Kong/F-Zero/Kirby Air Ride would generate. From both a consumer and business standpoint, MK9 should hold more priority than most other games let alone just racers.
Then why haven't they released multiple mario karts every generation? We know the answer, because they would just compete against each other. And most people who bought the first one wouldn't bother with the second one because they already have the Switch Mario Kart. Mario Kart isn't gonna happen. Deal with it. Mario Kart 9 will probably be Switch 2 launch title. It makes no business sense for them to try to cannibalize their best selling game with a sequel on the same system (which they've never done) that would no doubt be very similar, rather than spend those resources making a different game that will sell on its own and not cut into the sales of their biggest seller. The only people who want Nintendo to completely change their Mario Kart strategy and release two Mario Karts on a single system are some of the Wii U owners who already had 8 on the Wii U. Needless to say thats a very small percentage of Switch owners and Mario Kart 8D owners.
Mario Kart 9 isn't happened on Switch. It would be a stupid decision and would be totally changing Mario Kart from a one per system series. There would be no point in releasing another mario kart for the same system. Not gonna happen. Nobody wants this. What would you rather have, another very similar mario kart that we already have on the system, or a brand new game? 99% of people would prefer a new game.