One of the links in my OP leads to this: http://www.democraticunderground.com/12511829582.

We already know racism inflicts many older white folks. It's the surveys of white millennials that are more surprising and worrisome.

Bill Clinton was certainly popular and that may have helped Hillary Clinton make the 2008 race fairly close. The bottom line is that winning the Democratic Party nomination without strong support from POC (especially those who actually vote) is nearly impossible (when the electorate is increasingly diverse). And I think a very large and diverse coalition is key to bringing about systemic change, which won't happen via a top-down approach.

I think Bailey makes many excellent points, and one point I'll add is the perception of electability. Obama wasn't immediately viewed as electable, but once he was, he started winning over POC and others. Sanders is not viewed as electable by a sufficient number of people, and I think that's pretty understandable. At the very least, it's understandable that people aren't willing to take a chance on nominating Sanders when the Republican threat is so great.