I posted this in another thread but it seems like it would be good to post here as well:
A) Historically low awareness among them. Clinton was not only 1st Lady (which gave her a lot of exposure) but she and Bill have been very active in interacting and mingling with the minority (especially Black) community recently and for decades.
B) Doesn't appear to put race issues at the forefront of his campaign. Even after the encounter with BLM and the update on his campaign site, when pressed on race issues he still tends to pivot back to economics - its clearly what he's comfortable with. He can also get kind of belligerent and exasperated when pressed (can he handle the pressure of a Republican controlled Congress cockblocking him all day, every day?). I remember a moment when one of his staffers apologized to BLM on his behalf and he later vehemently took that apology back himself. Wasn't a good look.
C) He's the former Governor of Vermont, one of the Whitest states in the country with numbers that don't look very good for minorities when you look at employment, imprisonment, etc. Makes a lot of people skeptical of how well he can hear and fight for minority issues (and before you counter, yes a lot of people are aware of the Clinton's "tough on crime" stance in the 90's and weren't fans of how that ended up either)
D) A lot of Bernie fans (even on GAF but especially elsewhere) are awful and either subtly or overtly racist. Often times when critiques come from the direction of minorities, "Standers" try to deflect with "Bernie marched with King 50 years ago, what more do you want!" or "Don't you know what's good for you?! If you don't vote for Bernie you're stupid!", etc. Basically calling themselves allies of minorities but when they speak up they get shouted back down by these people. I'm certainly not saying all Standers are like that and you saw something similar in Hiliary's camp back in 2008 but there are enough of them and they are loud. Whenever I see stuff like that (and others I've spoken to feel the same way) I get REALLY turned off from Bernie.
E) Electability is more important for the minorities. White voters can afford to go "all or nothing" because if Bernie loses the outcome won't be as bad for them. That's why you see some Standers say they would rather vote Republican or not vote at all if Bernie doesn't get the nomination. For Black people and other minorities, a Republican victory (at least from the pool of candidates we have) would be a doomsday scenario. They feel they can't afford to lose at any cost so rather than go all "pie in the sky" they are more pragmatically focused and are more interested in who they think can win and bring some progress than someone who
might bring more progress but has a harder chance of winning. Rather aim high and win than aim for the stars and get nothing or slide backwards (again assuming Bernie represents all that is good and pure in politics

).