Pokémon Sword & Shield represent a huge jump for the Pokémon series, as they are the first mainline entries in the series that are being developed for Nintendo Switch. The move to Nintendo Switch means that there is more pressure on Game Freak to deliver a title with graphics that are appropriate for a home console. The fact that Pokémon Sword & Shield are the first games of a new generation means that the games will need new assets for the Pokémon themselves, too. The early trailers for Pokémon Sword & Shield showed beautiful graphics for the characters, Pokémon, and the Galar region, which made the fans excited to see all of the Pokémon represented in Pokémon Sword & Shield.
It seems that fans of the less popular Pokémon will be out of luck in Pokémon Sword & Shield, as Junichi Masuda of Game Freak revealed during Nintendo Treehouse: Live and to Famitsu magazine that not all of the Pokémon from the series will be available in Pokémon Sword & Shield. The Pokédex will include over one thousand Pokémon in Pokémon Sword & Shield, so Game Freak made the decision to only include a finite number of them in Pokémon Sword & Shield, which means that players won't be able to trade Pokémon that don't appear within the game using the upcoming Pokémon Home app.
Masuda also stated that future Pokémon games will follow the same pattern and will only include a specific number of Pokémon in each game, rather than all of them. Masuda didn't rule out the possibility that cut Pokémon and features could be included in future updates for Pokémon Sword & Shield, but those decisions have yet to be made within Game Freak. Pokémon fans are naturally incensed by the removal of Pokémon from the series, as there are people who have been using the same team since Pokémon Ruby & Sapphire and have been trading them forward in each new generation. It has yet to be revealed which Pokémon have been cut and how many Pokémon will be appearing in Pokémon Sword & Shield.
It's possible that the backlash could convince Game Freak to add the cut Pokémon in future updates, but they definitely won't be there at launch. The fact that Animal Crossing: New Horizons has been delayed to 2020 means that Pokémon Sword & Shield are the Nintendo Switch's big holiday 2019 titles and it will take a lot more than some backlash on social media to cause them to be delayed.