>>39681571Because the games were developed with a specific set of PKMN in mind instead of following a bloated checklist of all the hundreds of things that need to be in the games.
A Furfrou hairdresser, a location with Moss Rock and Icy Rock, a magentic location, an Alola-atmosphere area, 10 evolutionary stones, 17 unique trade-evolution items, the ability to "turn your 3DS upside down", 9 incenses, a bunch of NPCs giving you a shitton of special items like 17 Arceus plates, Primal orbs, Genesect drives, 17 Silvally discs, Kyurem items, SM-mascot items, Kami mirror, Oricorio nectars...Deoxys meteors, Rotom appliances, a Zygarde cube, Gracidea flower, a Meloetta NPC, a Dragon Ascent relearner, a prison bottle.... a key stone and some way of obtaining every single Mega stone for the 4th time.
But that's just annoying details.
The other thing is that the moment you allow people to use ALL old PKMN, you've opened competitive scenes to the same tired dozens of OP legendaries as previous generations, whereas restricting the number of legendaries to a small group of those relevant to the most current game actually makes them feel at least somewhat special, while the old ones fall into myth until a game comes out where it'll actually make sense for them to exist again.
