There is no logical explanation. Even the online excuse is just that, an excuse, and a lame one. Some higher ups at Take 2 Rockstar are obviously plain stupid and/or still butthurt over the situation at DMA from over 20 years ago when Nintendo treated them bad. Once these morons retire is the time when a change can happen.

A single player DLC would've sold millions too. They clearly couldn't care less about the single player part of GTA5 though, regardless of whether it's content for the existing version or new ports, unfortunately.

Seems unlikely to be a technical or financial reason at this point. Maybe they have a legal agreement that prohibits them?

I don't really think that all games should be on all consoles in general, though. Each console should develop its own distinct games library.

Developing for the Switch would divert resources from GTA VI or other games.

While Switch is higher tech than PS3/360, developers prefer to work with the newest tech available. So being less powerful than PS5 or XB Series would discourage them from porting to Switch rather than PS5/XB.

Switch being different infrastructure also makes porting annoying.

There are studios that exist just for ports. Rockstar could have easily outsourced the port if they really wanted to, but they don't care.

That budget could be used for other games to be ported though, rather than the 10 year old game.

Fair, but they haven't done that either. Unless you count the awful trilogy port, or L.A. Noire all the way in 2017. Plus, it's still GTA V, the game keeps reaching the top 10 weekly sellers to this day.

I'd blame Switch's poor online service. GTA V is all about GTA Online for Rockstar, it's where it makes most of it's money, from it's Shark card microtransactions and such. Hopefully Nintendo has much better online planned for Switch 2, if they do I would imagine it may finally get a GTA V port, and possibly even GTA 6, since GTA 6 is supposed to release sometime in late 2024 or 2025 according to insiders and Switch 2 should be out by then.

To be honest, that's honestly not too bad. Switch is almost the same as XBox, and XBox is heavily driven by 3rd parties and gets almost every major 3rd party game whereas Nintendo doesn't. 

That's still a sizable chunk of the market, Nintendo just needs to make it easier to port games if they can, that to me seems like the main hold up (it's not worth the squeeze). 

And the counter point too would be that the Nintendo 3rd party figure is held down because they often don't get a lot of the major 3rd party releases. If Resident Evil 4 Remake was on Switch for example (putting hardware limitations aside), I'd buy it, so would probably a lot of people. 

Those numbers are with Japan though where Xbox is almost non existent. Without Japan, those numbers would probably be closer to Xbox: 30%, PS: 55%, Nintendo: 15% (Guessed of course but obvious that PS and Nintendo would be down and Xbox up in return). Doesn't explain why major Japanese titles are skipping Switch (it's mostly for technical reasons as you already mentioned) but it's certainly a reason for major Western 3rd party games (hardware remains an issue of course).

Btw. RE4 Remake isn't available on Xbox One either. Smaller userbase than PS plus worse hardware.