Yasuo vs. Riven made complete sense, as both champs had the reputation of having a large amount of "toxic mains" and the event was a fun way to incorporate the rivalry and give a nod to the community at the same time.
But Garen vs. Darius? Maybe I'm missing something, but this is how I see it.
1) Darius is generally considered more popular across the board. There's no real community rivalry in comparison to Riven and Yasuo.
2) Darius had slightly better rewards (a chance at a second skin instead of a chance at extra orange essence). So people who didn't care and were aware of the reward difference would always pick Darius.
3) Darius is a counterpick against Garen, so people who don't follow the lore at all would most likely take Darius anyway.
1) Garen isn't a king. That is Jarvan's shtick.
2) Darius is currently a rebel with a dictator-to-be track record (conquering his homeland to rule it how he sees fit).
3) Garen isn't a dictator. He's aware of the faults of his kingdom and fights for the good in it, believing that by protecting it he can enable the next generation to do better (or that's the feeling given off by the lore).
This poor storytelling feels familiar... did Warner Brothers have anything to do with this?
If you really wanted a vs. event, I have some suggestions that at least seem to be less one-sided and oddly written.
- Lux vs. Morgana (light and dark)
- Leona vs Pantheon (order and chaos)
- Vi vs. Jinx (order and chaos)
Just some suggestions; at least they'd be able to stay in character.
But who knows, maybe I'm the only one who just doesn't get it?
Edit: Thank you to Rioter @Carnival Knights for answering this, and to everyone else who responded as well. I now have a better understanding and appreciate that so much! I was also wrong on some things, but hey, that was the point of the post: to gain a better understanding and learn. Thank you all!