I'm not that surprised. It's possible technically but to get the game to fit on a Switch cart, of which they'd have to use the most expensive format (which ups production costs), against Online being pretty compromised and more sparsely engaged - if not absent (which is where they make the most money), makes the investment seem less rewarding. And they probably don't want to do a cloud version or have the digital filesize exceed the actual storage of the Switch.
Rockstar is also pretty bad at compression on Switch, the trilogy and LA Noire are some of the Switch's biggest games, size-wise. Well, they didn't do the ports, but for the trilogy at least, they didn't seem to care enough to delegate to a competent dev, or allocate the budget to make it less rough.
I do think GTAV will show up on the next thing though. But then again, I don't think GTAVI will. I think Nintendo is just gonna have to stay one GTA behind. Might catch up on Red Dead, though.