If you really think about it a lot of the old Pokémon are irrelevant to kids today. Gamefreak knows that Pokémons target audience is small children. Most of the people who grew up playing games like Diamond & Pearl (Games that came out 12 years ago) have moved on from Pokémon to other games like Fortnite or just have stopped gaming in general. A lot of kids would be happy with just having 150 Pokémon in the game and just catch whatever they find in the game.