People are individual survival machines, that's why. Each bastard in the Omelas knows at the gut level (not in some abstract way) that there's a child being miserable specifically for a tiny bit of his happiness. His personally. He will then kill for larger bit of his happiness. He isn't society. He's an individual. It is all between him and that child. At very best, between him&his family, and that child. The society ain't part of equation. (And if it is, the communism should of worked perfectly in that universe) [assuming that the individual believes he won't be caught]

edit: also i think you don't understand the story. They didn't take the child apart for much needed organs to save other folks in Omelas. The child is miserable for the purpose of bringing sense of unity into the commune, for the purpose of making them value their happiness. That is already very irrational, and not only that but also entirely contrary to how homo sapiens behave when exposed to gross injustice.

edit: To explain my use of language. We are not talking about rational agents and what they ought to decide. We are taking of irrational agents that are supposedly (premise of the story) made more well behaved by participation in a pointless and evil ritual, which is the opposite of the known effect of direct participation in that sort of ritual, on populace. That's why the story makes a poor case against utilitarianism. Because the consequence is grossly invalid.