The easy explanation as to why Persona hasn't made it onto a Nintendo platform outside of the Q spinoffs is because there was never a real platform that facilitated them. Post-SNES Nintendo platforms were never JRPG friendly, and Shin Megami Tensei wasn't always cordoned off to being "the Nintendo series" either because Nocturne and Digital Devil Saga were both Playstation exclusives - the former of which was essentially the jumping off title for Persona 3. Handhelds thriving on the other hand, gave them more of a reason to make SMT their "portable" RPG series, and the developers of SMTV have already mentioned in particular that the reason they went with Switch was because of the portability factor. You can argue that up until Strange Journey, the SMT franchise had just as much "Sony association" as Persona currently does.
That's not the explanation, the explanation is because that's their actual intended business strategy. Like I said, they had a survey that even talked about this in one of the questions (it was a question about how people feel about them not doing multiplatform development). They had several opportunities through their whole story to change to multiplatform games and didn't, it's just not how Atlus operates.
The difference between the Platinum situation is that Platinum actually does make multiplat titles. Nintendo promotes and even funded/published Atlus games before that are made by a team trying to reproduce Persona's success. If they really wanted Persona, wouldn't make more sense to pay Atlus for it instead of developing as a new IP?
Outside of that, things are different now, Switch is a platform that's thriving in Japan, and more Japanese titles and JRPGs are making their way onto it. It's not unreasonable to assume that Atlus and Nintendo might eventually want to put a Persona title on there. Just because Nintendo made one exclusive title together with Atlus doesn't mean they wouldn't want more, especially if that might sell. If there is some hidden deal that prevents this, I encourage you to share it.
If things were different, SMTV would be on PS4 as well. It isn't tho. Atlus didn't change anything about their strategy and they absolutely never said they did. There's no Persona for PC, they block share mode for their games, they still developed several games for 3DS and even Vita pretty much after the systems "died", didn't develop even a mobile game for Sega. It all shows a very conservative company, not one willing to change their whole business strategy over a fighting game character -- I 100% doubt they would even want to use a character not from their Nintendo teams, even more when they want SMT to be bigger.
There's no hidden deal, but anyone expecting them to change their decades long business strategy needs to actually show proof of them intending to change and SMTV's exclusivity pretty much spells that they're still the same, not even adding Catherine Full Body into the equation -- a game that is coming to even Vita, but won't release for PC/Xbox/Switch. The whole Persona on Switch thing reads more as wishful thinking than anything, Atlus never even showed any interest in doing that. Switch will still get mainline SMT, Takada's team games and Etrian Odyssey; PS4/5 gets Persona, Catherine and the new Hashino games.