THEY PICKED KALISTA

They both focus on kiting, through stealth and infinite dashes, and tank killing. Vayne does more damage, but is at much higher risk, and usually has a much weaker laning phase, though that depends on the supports and enemy ADCs. Kalista has more DPS but less burst, Vayne just ends up with really high burst, but once again, really high risk.

It's possible to win with anything(Yorick, Galio, Urgot, Mordekaiser, etc. ),but in certain metas, there are more optimal choices. Right now, for the current meta, Vayne is the optimal choice. She has also been getting buffs from Riot as of late too. Another similar comparison would be Varus and Ezreal. You want poke, but right now, almost everyone has a slow, be it through Iceborn Gauntlet, Frozen Mallet, or Rylais. Ezreal has that. Ezreal has his E too, providing him with more safety nets. Champs without any mobility do poorly in this meta, due to a lot more mobility and cc being tossed around(frequency of slows with aforementioned items). Giving up after one game of a champ isn't being a meta sheep; When I play a game with a champ the first time, I get a feel for their gameplay style. For me, Braum didn't fit my style of gameplay. I played one game with him, and stopped. If a champ's playstyle fits yours, you tend to do better with them. Kalista's unique playstyle is only for the diehards. You HAVE to master Kalista to do well with her. You HAVE to master Riven to do well with her. With things like Yasuo, Zed or Lee Sin, you can master them to do better, but even if you're new to them, you don't feel like a useless sack, because they have lower skill floors.