I saw a few posts on Twitter that amplified the "Why Bernie?" theme too. It's because he's specifically running as the socialist candidate that creates the opportunity for this discussion. Socialism shouldn't be some trickle-down benefit that starts with white people--and white Bernie supporters are wrong to think that socialism is this kind of simplistic mechanism.
Attacking racism would put a lot more progressive ideas in the mix, since Blacks tend to be more progressive than whites, which is shown by numerous polls in the US.