I honestly feel like this is a culmination of all the negativity that had been brewing since 2014. XY was viewed as terrible (rightfully so) with how shallow and insignificant the story was, how the gameplay was neutered in terms of region design and any semblance of challenge (though, online was significantly improved with PSS). And then you add in that post-game was cut, but people understood that the post-game of every new gen game was always neutered. The major problem though is that it came right after Generation V, which had a great story, tons of gameplay whether it was in post-game content (specifically B2W2) or in the Unova region itself.
Then ORAS happened, and people expected that remakes would add in a whole lot of content that the first Gen games didn't have. And then news started to break that the Battle Frontier was reduced to a tongue-in-cheek reference, all because of some misguided belief that people are playing less of their content due to mobile gaming (when has this ever been the case? Pokemon hasn't dwindled in popularity. This is something I will NEVER understand). This is when there was a true break between what the developer expected vs. what the fanbase expected.
Pokemon Go, and Sun and Moon was a step forward, but Sun and Moon in particular did have a few steps backwards. The overemphasis on cinematics resulted in a shallow story feeling even more shallow (with the addition of wasted time in cutscenes that you couldn't skip, and cutscenes that patronized the player by consistently telling them where to go). The story was much improved from XY, but some of the problems still remain: region design was brain-dead in straight lines, post-game content was meh (Battle Tree is just the same thing but with more nostalgia due to Pokemon Go + Gen I references).
Add in the whole Dexit controversy that was oddly revealed in a Treehouse segment, and it became the proverbial straw that broke the camel's back.
