Pokemon Sword and Pokemon Shield's newest trailer gives credence to the now-longstanding "Eviolite" Pokemon evolution theory. The theory posits that Game Freak and The Pokemon Company won't add new evolutions to existing Pokemon due to the existence of the Eviolite item. Eviolite, introduced in Pokemon Black and White, grants additional defensive stats to Pokemon that aren't fully evolved. As such, any new Pokemon evolutions could imbalance the game by granting Eviolite access to previously evolution-capped Pokemon.
This Eviolite theory is proven accurate once more as a result of the new content shown in Pokemon Sword and Shield's new trailer. In the trailer, Game Freak revealed the new Galar regional forms of Zigzagoon and Linoone, as well as a new third evolution named Obstagoon. Going by the logic of the Eviolite theory, Game Freak couldn't add Obstagoon as a new evolution for Linoone. Instead, Game Freak created all-new Pokemon via regional forms for Zigzagoon and Linoone, from which to add the third evolution.
Further evidence of the Eviolite theory may be forthcoming soon. An increasingly accurate leak regarding Pokemon Sword and Shield's starters also has information about a new Farfetch'd evolution named Sirfetch'd. If the Eviolite theory is accurate, then instead of making Sirfetch'd a Farfetch'd evolution, it will make a new Farfetch'd Galarian regional form and then evolve that into Sirfetch'd.
This theory still has plenty of gaps in its logic to fill, but remains foundationally accurate. Game Freak appears to have a rule that existing Pokemon can't get new evolutions, put into place in the fifth generation of Pokemon titles when Eviolite was introduced. Whether Eviolite actually is the reason for the rule isn't clear. It could be that Game Freak doesn't like the idea of adding new evolutions to old Pokemon.
Just goes to show that the more Pokemon changes, the more it stays the same. Game Freak's iterative approach to improving on Pokemon from generation to generation is slow-moving by design. Maintaining certain traditional aspects of the game is as much of a priority as introducing new features. Sometimes this can be limiting, but Game Freak's clever design doesn't let that slow it down.
Pokemon Sword and Pokemon Shield release November 15 exclusively on the Nintendo Switch.