If Nintendo pay them to.
That's how independent devs who aren't big enough to self publish operate. They need somebody to fund and publish the game. For them at the moment that's Microsoft, so unless Nintendo were to offer to pay them for a game, they're unlikely to make one for Switch.
Bet Shiken that COD would outsell Battlefield in 2018. http://gamrconnect.vgchartz.com/post.php?id=8749702