Zed isn't considered a steong team fighter. He is a duellist and a split pusher. In korea the meta generally takes the primary shape, and sort of branches out to other regions, with aome minor variation. (Where lissandra has like a 90%~ win rate but around 60% in other regions. Kalista being primarily a NA carey not seeing as much play in other regions)
the current meta game goes in Korea its heavily favored for mid game heavy enguage team fight teams. Zed being a poor team fighter in the eyes of korean players has brought him down. Along the same lines there are a lot more favorable AP match ups against zed (leblac, ahri to name a couple) who can do zed things much faster, with less itemization and have better niche plays. Ahri can vlow up teams, and is more mobile. Leblanc with good positooning can pop most mids, all supports (except maybe nautilus as of late) all ADC'S most aggressive junglers, and all tank junglers at early stages. So she basically makes games 4v5s when played well. Ahri can navigate team fights and dive as a secondary with the front lines and is still mobile enough to walk out.
Tl;dr there are a lot of match ups that can beat zed, and koreans dont split pusb
