Clinton is basically running to be Obama's third term. She's selling herself as safe, pragmatic, competent, etc. She's the "knows what she's doing and can get things done" candidate. She's going to beat Sanders unless he can offer something new and appealing.
Sanders is running a much more emotional campaign. He's identifying Bad People who must be opposed. He's proposing radical change. His supporters are less concerned with policy details than they are with knowing that "he cares about people like them". It's important that he's perceived as honest and authentic. If he doesn't seem to care a lot about you and your problems, you're not likely to give him a second look. It's not about being sure that his policy prescriptions are really the right way to go; it's about believing that he's on your side.
Sanders is clearly very willing to signal that he's on the side of, say, young white people with lots of student debt. This is who he's aimed at. He's said a lot of things that "everyone knows" are stupid politics, so they trust him. He has not been similarly willing or enthusiastic about sticking his neck out on behalf of black people, among others. I suspect that his "cares about people like me" score is much lower among black people. And reparations is part of why - it's the sort of costly signal that a Sanders-like candidate would be sending to black people to prove that he's on their side. He's vulnerable on this in a way that Clinton isn't, even though they have the same (non)position, because his whole campaign is about sending these sorts of "I care" signals.