Personally, a reason I fell out of playing the Pokemon games around Generation III is that they got stale. Choose from three starters, get eight badges, foil a mafia/cartel/terrorist organization with gimmicky name, become the Pokemon League Champion, and maybe uncover the secrets of a legendary Pokemon, which leads you to catching it.
I wanted different Pokemon games. I’m surprised we never got a proper fighting game, where you directly control or move the Pokemon in the fight. I am surprised we’ve never had a story with more mature themes or tones, something that was more of an epic than a kid trying to hit it big by doing something trendy.
I also got annoyed by the increasingly low effort in developing new Pokemon. With every generation, the designs seem to get uglier, and the concepts seem to get sillier. I feel like the creators don’t have the same heart in it that they did in the 90s.
Also, this is something that really made the original games special: the bugs and glitches, as well as the rumors that circulated by word-of-mouth before the Internet was what it is today. There was this mysticism about the game that just does not seem to exist anymore. Even Pokemon Go fell short, not being very much like what the commercials implied it would be.