Because black voters don't make votes based on fucking over white folks. We quite literally don't give a fuck about white folks especially not enough to vote to systematically destroy them. We don't give a fuck about them to destroy what little of what we have just to "stick it to them" White America quite literally isn't in our purview when we vote. We vote for at the very least to prevent things from getting worse for us, and with the hope that shit will get better for not just us but our kids. The irrational hatred of black folks on the other hand are the reason for many white republicans voting patterns (study after fucking study, after fucking study show this but America keeps denying it).
People roasted Killer Mike because he was trying to convince black folks that fixing economic issues would fix racial issues. People started roasting Killer Mike because we were tired of him being summoned much like MLK as a pokemon to beat black folks over the head. Killer Mike don't represent black people anymore than Justin Bieber represents white folks, but time and time again. Every time white folks get even one black person who agrees with them that person is suddenly President of Negroes and his voice is our will and he/she is trumpeted out at every opportunity.
Hillary took a different approach, she got celebrities sure but she didn't get them specifically to speak to black folks. She brought out Lebron to a damn white Ohio crowd. She brought out Jay Z to a predominately white crowd. When she wanted to speak with black folks, she held smaller venues and had people from the communities speak out. When she decided to speak on policy brutality you know who she brought to let them speak? Mothers of men/women/children who were killed by police. Not a rapper, or black actor. Actual people in the community who were directly affected.
When you compare and contrast yes Killer Mike looks like a prop, an item on a checkbox. Imagine if a politician was talking to you about heroin issues affecting your community and they brought out Katy Perry to speak to those affected because hey she's white, you're white, she's well known by people who share the same skintone, good enough!
Bernie should have completely stole Hillary's entire approach and amplified it to make Hillary look like she didn't give a single fuck. It also helped that the people in charge of Hillary's outreach to the black community were black women not single woman, black women. Who better to do that? You know who made the Killer Mike decision? A white guy, I'm sure he means well but at the end of the day his intimacy with the black community can be narrowed down to watching "12 Years A Slave" a handful of times, reading a few snippets of some MLK speeches, and hanging out with some black dudes in college.
Bernie also should have taken his ass to the South, as others have pointed out he made the EXACT same mistake as Hillary did in the midwest, much like everyone loves say when it comes to the midwest "why should they vote for her when she didn't even take the time to go there." The same question should be asked about Bernie. When in the fuck should the black community vote for Bernie when he couldn't even be assed to show up versus Hillary who not only showed up but went directly to the black community and asked and listened to their issues directly not via some rich proxy?
Honestly the shit ain't hard, you want people to believe you're for them? Show up and prove to them you are. Having some celebrity who shares their skintone and having marched with MLK in the 60s isn't enough. You gotta grind for that shit, it ain't gonna be handed to you. If you face an opponent with and advantage you don't go "whelp fuck'em" you grind even harder.
Black people in this country don't owe a single white person breathing or otherwise a damn thing. Bernie wasn't owed their vote because he had some black celebrities. For as trash as I think Hillary is, I'm definitely not going to sit here and suggest that Bernie should have gotten the black vote over her when she did WAY more work for the black vote, doubly so given her super predator/crime bill bullshit. And on that subject at least she apologized for that shit. Bernie cosigned on that same damn bill then fucking double downed on it on some "single voter issue" type shit. Like "sure it fucked over millions of black folks, but that assault ban tho!"
So no, black voters and white republican voters aren't anything alike.