The biggest hurdle is most likely that Final Fantasy XV was developed on Square Enix’s own internal Luminous Studio which currently does not support Switch. This is a similar issue with Resident Evil 7, developed on Capcom’s RE Engine. Last year, Final Fantasy XV Director, Hajime Tabata, said they attempted to get Luminous Engine running on Switch but they were having issues. We can pretend this was a long-con to make it seem like they tried. Occam’s Razor suggests porting the engine probably wasn’t worth the effort.
I believe the intention was to get the original version running on Switch. Effort acknowledged. Sometimes it doesn’t work out. Final Fantasy XV Pocket Edition was also announced for PS4 and Xbox One so I’m not of the mindset that Square Enix is merely serving Switch owners scraps. The company didn’t have to port anything to Switch so while I’m not interested in this at all, I’ll give credit where credit is due. The power disparity between Switch and other platforms makes porting a minefield.