Combination of problems here.
1. Sanders personally didn't run a good campaign in SC apparently. He has limitations. He has no longstanding ties to black voters as a support group.
2. The Clintons are liked by most black people. This is hard for lots to get their head around but the 90s were a great time for a lot of people economically. Many black people were lifted out of poverty by the 90s economy. And despite accusations of economic reductionism by sanders, the economy is still the most important issue to black voters and all voters. The issue here is that I suspect longing for the boom of the 90s to return superseded sanders welfare statism with SC blacks.
3. The nature of SC politically was bad for sanders. It's a state with next to no organized labor or independent political grassroots type organizations. Much of the democratic machinery is wrapped in old white party loyalists clubs and the black church. Sanders is not a democrat and thus has no ties to the old local democratic clubs and he had no inroads with the black churches. This is a divide that is exasperated by both sanders geographic isolation and cultural Jewery/spiritual Atheisim.
Clinton on the other hand has great connections within the remnants of the white southern Dems and the black church. The Clintons are southern democrats to the core, they understand how to manage their client groups and have been since the 70s. Clinton is of course also a Methodist and a southern, these matter when it comes to communication.
4. I think HRC's embrace of Obama was atleast partially successful. Black folk, especially older black women, love Obama. Exit polls showed that the perception of disrespect towards Obama on Romney and the GOP's account drove up black turnout and was a primary reason for many black voters.
By cynically painting Sanders as undermining Obama, HRC atleast partly tapped into this. Sanders personally failed to deflect this criticism well. Partly because in fact he and Obama do have pretty large disagreements on matters of policy. And despite sandera not really being an undermining force to Obama's incredibly modest progressive goals, he refused to disavow these differences. Though sanders could have pushed HRC harder on her own differences with Obama on foreign policy, she had an excellent defense of this in that despite these, Obama made her his SoS.
Though I dismissed these at the time, I think maybe cornel west's presence and prominence as a Sanders surrogate wasn't helpful on this front. West's flair for dramatic language has perhaps made him toxic even in the halls from whence he comes. I'm sure his declaration of John Lewis being a neoliberal sellout(which he sorta is, but decorum and all) will not be a helpful thing going forward.
5. I'm sure there are a fair amount of moderate, neoliberal black voters that just prefer those policies.
6. In the end, there are still bright spots. Sanders did better with younger voters. Only losing millennial blacks 45-55(compared to losing over 65 by ~95-3). This indicates that left economics remain a winner with the young across race. This indicates to me that a left candidate not as uniquely weak as sanders in these respects would do better against a moderate, establishment candidate not as uniquely strong in these respects.
The problem with this conclusion is that I currently see no left inheritors of Sandersism that has much stronger variables here. A Warren is marginally better for sure in a number aspects but not decisively. Someone like Keith Ellison has separate cultural barriers, tho he has the unique advantage of being black. Whereas the neoliberal establishment has thoroughbreds like Cory Booker at their disposal in the future.
However, the political landscape is ever changing. In 5 years, from Generalissimo Trump's Very Luxurious Day Spa/Internment Camp, the whole situation may be different.
. What he would've done was a quid pro quo, and Bernie's not about that life, which is why nearly every senator endorsed Hillary.