This is a bit of a devil's advocate move, but if I was a black man that had driven around a young white girl who later disappeared, but I couldn't prove that I was not the one that did it, I might be hesitant as well. Yes, good people come forward, but an innocent person might be TERRIFIED in that situation especially if they are a convenient, easily convicted target. I actually joke about it with my boyfriend. Like, if he left my house one night and then someone came in right afterward and killed me NO ONE would believe it wasn't him. NO ONE. He said he would just accept his jail sentence and then still look for my real killer while in jail
Now, obviously, my boyfriend would have to come forward because he would be suspect #1 immediately. However, I can understand having an "oh **** this is going to be pinned on me" moment or being scared to come forward even as an innocent person.
However, I think it most likely was him, all things considered.