(1) Blacks are actually very conservative, which I keep saying, so they are not attracted to the more liberal candidate. And no, this does not mean they are going to become Republican Ted Cruz supporters, they will vote for the party that gives them stuff (like affirmative action and various welfare programs) and that they can boss around.

(2) Blacks are low-information voters, so they are biased in favor of the well-known candidate. They are not going to put in the effort to learn about Bernie.

(3) They vote for whom black leaders tell them to, and black leaders are lined up behind Hillary (because the Clintons are way better as schmoozing them).

(4) As noted by many commenters, a nerdy old white guy is not especially attractive to black voters. An old white woman isn’t all that attractive to them either, but Hillary has the first three points strongly in her favor.