Gen V was probably the best Pokemon ever was in terms of content and effort. Gamefreak really poured it's heart and soul into making the first non-Japan based region to make it as lived-in seeming as possible. And their reward for all that effort was the worst sales and people being grumpy about old gen mons being post-game locked. So Gamefreak never put in that same amount of effort ever again. And then Sword and Shield became the most sold games since Gold/Silver, cementing that lesson.