Why is John upset at not being allowed to be the sacrifice?

John is upset because he cannot be the sacrifice.... he has the wrong color skin. What upsets him most is that he is the best qualified.

"I ought to have been there," the young man went on. "Why wouldn't they let me be the sacrifice? I'd have gone round ten times–twelve, fifteen. Palowhtiwa only got as far as seven. They could have had twice as much blood from me. The multitudinous seas incarnadine." He flung out his arms in a lavish gesture; then, despairingly, let them fall again. "But they wouldn't let me. They disliked me for my complexion. It's always been like that. Always." Tears stood in the young man's eyes; he was ashamed and turned away.