Mary Dalton and Bigger cannot have anything vaguely resembling an ordinary relationship. The problem is that there are just too many assumptions and misconceptions on both sides.
Mary befriends Bigger, not because of his personality or any qualities he might have, but purely and solely because he's black. As a self-proclaimed progressive, Mary has blurred the personal and the political to such an extent that she cannot see Bigger as a human being in his own right. Because she sees Bigger exclusively through the prism of radical politics, she can never establish an authentically human relationship with him.
Despite her theoretical commitment to racial equality, Mary has inadvertently guaranteed that Bigger will continue to feel like a member of an oppressed minority in her presence rather than a man. This is due to the fact that Mary has reached out to him because of his race, not because of his humanity.
For his part, Bigger cannot get over the fact that Mary is not just a privileged white woman but also the daughter of his employers. This means, among other things, that he can never truly be himself around her. He's always worried that he'll get into trouble over spending time with her, running the serious risk of losing his job—or worse.
He may live in Chicago and not the Deep South, but racial prejudice is still rife in this area, and Bigger feels its crushing weight upon his shoulders each and every day. But Mary doesn't see any of this. She's so blinded by privilege that she's incapable of understanding how her overtures of friendship—or comradeship, to be more precise—can so easily be misconstrued by white society.