I need to do a quick preamble on a very important topic, first, before I get into this discussion: Why aren't blacks voting for Bernie Sanders? Now after he's begun losing handedly in this demographic, people are now very concerned about why this is, and in response I think there have been some thoughtful responses in the African-American community, which I encourage people to read. The reasons are interesting and worth taking on board, but the reality is, it's too little and too late. At the core of it, there's a fundamental ideological difference between the two voting blocs that are most necessary to the success of the Democratic party. The first are Millennials, and the second are African-Americans/non-Millennial women.
Millennials have been taking it from the system since they got into the job market, they are "shockingly progressive," it's difficult to understate how irritated and mad they are, and it's also difficult to explain how anti-establishment they are. To contrast this, Hillary represents the Establishment, she's made no bones about how much of a part of the establishment she is, she openly takes money from Wall Street/Big Pharma/private prisons, and she openly will not tolerate single-payer healthcare, free tuition for state schools, and she will not break up the banks. That's a long winded way of saying that Millennials are wildly unexcited about Hillary and fundamentally view her as being "essentially no better than a Republican." On top of this, there is a pretty ubiquitous sense that if anyone other than Bernie is elected, there will be a second financial collapse. That means that for Millennials, Hillary is a pill that is going to be very hard to choke down, and this will cost her votes in the general election.
Blacks and non-Millennial women have different motivations, but the thinking is roughly the same: They want to see someone who they can affect a small amount of change and protect them from their extreme enemy, the Republicans. Particularly Southern Blacks (as per the article above) seem to feel that they are always on the losing end of the stick, so slow and steady change is preferable and sustainable. Women (e.g. in Texas) have been the target of many conservative attacks, and their abortion rights and rights in general have been a target of the extreme Tea Party craze of the last 6 years. Even though Sanders' policies would in principle help blacks and older women, there's a general sense that Hillary will handle this better and so they are willing to accept an ***-****ing from the establishment because for them, it's the lesser of two evils. And this general sentiment has very likely cost Sanders the primary, coupled to a set of other factors (i.e. The Establishment doesn't like it when you're against them).
Now, here's the problem. Democrats absolutely need both Millennials and blacks/women and Latinos to win the upcoming election. Those demographics are the key to the Democratic Party's success (If they get them, they will obliterate the Republicans in 2016). But here's the key problem: Clinton chose the nuclear option on Sanders' supporters as a part of her usual dirty tactics (Bernie bros, attacks on youth, etc) and the problem is that we are not thick enough to think that "Oh, well, Hillary didn't say those things personally, so I guess she isn't responsible for them."
This means that the Left, in terms of serious supporters, is now divided into two halves that are quickly becoming unwilling to work with each other, because they perceive their interests as being ideologically opposed to each other, and view each other's candidates as being wildly unsavory. I don't know what this means for the future, other than that there's about to be a massive war between the Progressive Left and the Neoliberal New Democrats. The Progressive Left is obviously going to win based on demographics, but that may take time --time that we don't have if there's a second Great Recession.