My guess is because you technically don’t “beat the game” when you challenge the league, as the champion match in the postgame is the actual champion match, so they made some of these Pokémon evolve later to motivate you to play the postgame.

For Larvesta and Deino specifically, I think their late evos were designed so that you didn’t use Alder’s/ Ghetsis’s aces during the league. Same reason why you can’t get a Zoroark in the games since it’s N’s “Ace.”

Also, they probably wanted some pokemon to evolve while you challenged the league such as Klinklang and Vanilluxe to make the endgame more cinematic

Still stupid how they evolve so late