Personally I feel like Volo has a higher bar to clear in order to access his battle since you have to catch every Pokémon, and that involves a bunch of side quests to get the rest of the plates in addition to going back for any regular Pokémon you haven’t caught. Lysandre just needs you to do the Xerneas and Yveltal side quests, and those just require one round of the Z-A Royale.
Volo’s battle is completely within the post-game. The main story climax doesn’t really involve him and is more about dealing with Kamado and Dialga/Palkia.
Didn't play PLA but don't you fight Volo and then he summons Giratina as the climax battle? If yes then Lysandre comes much, much later (aka "I am in the postgame and had no idea there was a Lysandre battle").
Volo’s battle is completely within the post-game. The main story climax doesn’t really involve him and is more about dealing with Kamado and Dialga/Palkia.





