Koo played a binary composition with only a single team fight threat - Jinx.
Kassadin counterpicks Lulu.
Rumble/Kassadin/Sivir is a tri-threat of team fighters.
Riven's biggest weakness is team fighting against burst/cc, and relies on split pushing, Kassadin can quickly gank and kill any pushed targets with minor map-play repercussions in the mid/late game.
TLDR: Koo picked a predictable and dumb draft. Lulu should've been flexed to top lane with a legitimate team fighting threat in mid-lane to give at least a 2 threat composition.