The Pokémon franchise has stayed basically the same since the first games. They've kept using the winning formula, adding more bells and whistles to the solid framework, but not changed a lot or expanded on its foundations. It works well, after all we love the games, and love to hate the anime, but there's only so much you can do to the existing formula without changing anything. And, seeing how constant the franchise has stayed, change would really upset a lot of people (imagine, for instance, if they replaced Abilities with something else or changed the base stats system). We get to a point where all they can add is bells and whistles of different colours. Perhaps something could be done with the linearity of the gameplay (having "defeat eight gyms in any order you desire" as a main stoyline, and have all the rest be included as a ton of sidequests, for instance), or changing the story so it isn't always the same (professor sends you out on quest, defeat eight gyms, beat evil team), but anything radical would make the fans go apedung with fury. The main series games are kinda "sacred", trying out new things is strictly reserved for spin-offs. And so it feels a little stale after a while.
Of course, we who have stayed with the series from the beginning are the hardest to please. We have seen everything they have done so far, and we crave that they always do better. People who buy a game and stick with it for a generation or two represent the ideal market, whenever they "fall off" the bandwagon, the same stuff could be sold to the next kid in a new package, but we oldies will complain loudly if we keep getting the same over and over. And, unfortunately for GameFreak, the diehard fans are responsible for most of the social media buzz, they will have to please us to stay in a positive light. No marketing department could ever do half the job the fans do when it comes to promoting Pokémon. If the games begin to appear stale to us, they quickly appear stale to the rest of the Internet too, and thus the consumers. But, again, if they change too much about the games, we go even crazier and would hurt the image of the franchise even more. GameFreak are walking a very narrow path, and it's understandable they don't want to take any chances.
It doesn't help either that we might be reaching eight hundred creatures this generation, meaning that old favourites have to be forgotten in order for new ones to get their time in the spotlight. There are also eight hundred 'mons to model, animate and texture (and write Dex entries for), demanding a lot of work to be done on the creators' part whenever they try out something new. Sure, with increasing cartridge sizes, the data can be held comfortably, but somebody still has to sit there and draw all those sixteen hundred eyes (give or take a few), or add shade to all the claws and teeth. Small wonder why they don't bother with realistic-looking fur and feathers.