Because, as said, its the laziest gen and makes near everything worse. The railroading is the longest its been in any of the games thus far, and the least disguised, AND the most rail-roady too.
The Pokedex is woefully incomplete. You are literally able to acquire Pokemon that do not exist in the Pokedex, as only the 300 pokemon Alola Dex exists, a national dex - standard since Gen II - does not exist.
We were introduced to a ton of great features with OR/AS; Super-Training, easy online play and connectivity, Pokenav/finder, and other bits and bobs that made it incredibly easy to do many of the more... Chore oriented parts of the Pokemon franchise. Are they in the latest iteration? No. Instead, they've removed half the functionality, and made the half that remains impossibly more complicated to perform.
The new Pokemon of this gen almost exclusively have a <5% encounter rate. Most pokemon you'll run into are from the other gens, which artificially pads out the game big time by hiding the pokemon that you're all excited to see so that you'll almost never see them without excessive grinding.
It throws you into battles straight out of a long 10 minute cutscene, with no opportunity to save. They haven't done that before, and by god is that just absolutely terrible game design.
The map is near non-functional, with how impossible it is to now tell the difference between walkable terrain/routes, and just random bits of scenery.
Lets not get into the horror that is SOS battle grinding, or goddamn weather battles.
The map is 100% disjointed and not interconnected. Several areas are 'connected', without anything to connect them, and on the larger scale each of the islands is completely separated from each other and not able to be travelled to without a ferry/fly. Even the original Ruby/Sapphire didn't do that - you took a boat, but were completely able to just surf that route later in the game.
The new fishing mechanics are legitimately terrible. 1% appearance rates are a fucking joke, and having to walk back and forth between areas to raise it to a still abysmal 10% appearance rate is just... Pathetic.
The list of complaints can go on and on. Sun/Moon is one of the worse Pokemon games, and is quite frankly just embarrassing when compared to its direct forbear. This doesn't mean its a bad game in total. Its still enjoyable enough, but it is a bad Pokemon game. Its getting picked out because of that.