Because in order to sustain a healthy, competitive environment, Pokémon company needs to put limits on things so that people don’t abuse it and take the fun out of the game. I don’t understand what you mean about “banning an entire thing instead of banning just the one move” but as far as I know, Game Freak never banned an “entire thing”. If you mean nerfs, I agree that the Parental Bond and Gale Wings nerf was necessary. If you don’t know, in OR/AS everybody in competitive used Mega Kangaskan, Talonflame, Xerneas, Primal Groudon, Cresselia, and that’s it. Literally no variety. I do think maybe they went a bit overboard with the Prankster nerf, but Prankster was annoying to deal with. As for the Burn and Confusion nerf, I think both were completely unnecessary and took some of the fun out of VGC. It certainly disrupted Klefki, as Klefki’s prankster swagger was now extremely nerfed. Anyways, I hope I helped explain why Pokémon Company banning/nerfing things is (sometimes) necessary.