It depends on how one defines "Left." Bernie Sanders is probably the furthest "Left" politician, according to how most people in the US define "Left," that could be said to be reasonably in the running as a possible candidate. We do have other parties that are much further "Left," going all the way to Communist/Socialist political parties and the like. But, they don't get much of a following.
Sanders has actually fractured the Democratic party a bit, simply because he is further Left than some Democrats would like. It also seems that younger voters tend to support such candidates, in my opinion. That's especially true when the candidate "speaks to them" using focused social media campaigns. Sanders didn't do as much of that as Ron Paul's campaign did when he ran. Ron is a Libertarian, not really Leftist, but the point is that those who appear to be successful on the Left in the US appeal to younger voters. And, young people don't typically show up to the polls as often as older people.