Usually the Grand Jury is presented the case by a prosecutor who is an elected official(sometimes Judges too). If this jury could not get an indictment even after the ME ruled this a homicide, I would speculate, I know it's speculation, that the elected Prosecutor didn't present a case worth indicting(See Ferguson's Prosecutor).
I dont know for sure that New York's prosecutors are elected but many are. This is a GROSS oversimplification but the main point is, voting matters in more ways than you think. And NOT voting helps out, how exactly?