why dont people play twitch?

Twitch's passive is no where near what Draven's passive was. Additionally, the rest of Twitch's abilities are awful compared to basically everyone else's.

Twitch has easy counterplay and lacks any early damage unless he snowballs. otherwise, there are a ton of ADC's with better objective pressure, teamfight ability, and general kit and damage. Twitch is great for catching people off guard, but with the rift herald now being present too and twitch having low mobility without Q AND no escape, team-oriented comps dominate more.

Basically, there are better adc's at the moment.