The more you look at this situation the more it devolves into a Mobius Strip of fuckery. But I'll try to break it down.
It could possibly be a combination of the two. Everything started on the 3DS as they had no faith for the Switch (which, after the Wii U I can't exactly blame them), but then they got word that there would be no more dedicated handhelds after the 3DS so they had an "Oh shit!" moment and hastily threw everything into an HD realm last minute, which is just asking for something to go wrong for a company that hasn't touched that kind of tech up until now. As for LG, the
coding for all mons are in it, but not the actual models themselves, so there was still porting to do.
However that brings into question why not all the Kanto mons made it (less than half actually) if they were all ready to go due to LG. It could be they didn't want to be accused of favoritism but then there's still all the Kanto whoring anyway so, who knows.
The
other other potential factor is that, after it became clear the full dex wasn't going to happen for whatever reason, TPC got the idea to use popular mons as extra "incentive" for people to use the Home service, which would explain the random Kanto cuts and why so many generally popular mons were snapped period, because then that would get more people to pay up for Home instead of if only the Lumineons and Wormadams of the world were cut.
Yet this
also flies in the face of the broken porter theory because that makes it sound like the cuts were planned all along, whereas the porter break implies they effectively played it by ear all the way. And there's always the outcome where for the next game they pull their own "Everyone is here!" to try and get back in the fanbase's good graces, and that in itself has questions as to if that was the plan all along and they just didn't think repeatedly saying "Dexit is forever now" would have repercussions (which is doubtful considering how they've been hiding away ever since the leaks happened) or as a response to the current backlash.
It's like, even
attempting to wrap one's head around this clusterfuck gives you an instant headache. I know I got one just typing this mess out.
EDIT: I just came across this. Oh boy.