Previous experience shows you need to get him up to at least a 70% banrate before Riot will admit an ad assassin is a problem.
Players don't ban rationally. Players will ban assassins (and other flashy, high damage champions) even when those champions have sub-50% win rates. They will fail to ban tanks and support champions, even when those champions have 55+% win rates.
Go to lol king right now. Yasuo and zed both have about 49% win rates and 50% ban rates. Sona has a 53.6% win rate, and a 2% ban rate.
Players (myself included) ban champions they don't want to play against, not champions that will most impact their chances of winning the game. Most players don't like playing against fed hyper carries and assassins. They will selectively remember the game where the fed assassin went 17/5, and forget the 4 games when that same champion did just ok, or barely contributed.
Edit: It's the same reason why auras were removed: players didn't value them as highly as they should have. Auras had to be extremely overpowered in order for players to think they were useful. They also tend to overrate single target damage (like zed, Vayne, khazix, etc.).