There's also the fact that Gen. VI introduced far fewer new Pokémon species than any generation that had come before it (which, admittedly, isn't such a big deal now, given what's been released since then, but was pretty major at the time), and the fact that X and Y were full to the brim of bullshit Gen. I worship, down to the game forcing the Kanto starters upon you, and them and Mewtwo getting Mega Evolutions when the Kalos starters and legendaries didn't.
X and Y were also made retroactively worse when it became clear that no Z version (or a pair of sequels) was coming out, and Zygarde was relegated to being part of a tedious and out-of-place sidequest in Sun and Moon. It just made X and Y feel even more incomplete and underwhelming than they already did.