I think, with a question like this, you should keep in mind that "The Ones Who Walk Away from Omelas " is not really written as a conventional short story. There isn't a clear narrative or sequence of events, and I don't even think there's really a clear setting...
I think, with a question like this, you should keep in mind that "The Ones Who Walk Away from Omelas" is not really written as a conventional short story. There isn't a clear narrative or sequence of events, and I don't even think there's really a clear setting (consider that Omelas is, itself, framed as an idealized utopia, one which exists in the imagination). Instead, Le Guin's story resembles a thought experiment more than it does anything else, and within the context of that thought experiment, the image of the mistreated child is really what the entire work hinges around.
Furthermore, I'd suggest you think about the style in which this story is written and the artistic intentions that lie behind that decision. In many respects, when writing about Omelas, Le Guin knowingly invests her imagined utopia with a sense of unreality. After all, consider that she herself, in that same story, suggests that this imagined place seems too impossible to be true, when she writes:
Do you believe? Do you accept the festival, the city, the joy? No? Then let me describe one more thing.
The imagery of the festival exists as part of this sense of heightened unreality which is embedded across Le Guin's description of Omelas as a whole. However, with the introduction of the child, this dream of Omelas is broken by the act of cruelty which defines and enables it.
So, with that in mind, we come back to your question: why does Le Guin not return to the subject of the festival? In response, I'd suggest you think about the structure of this story (particularly as a thought experiment) and the role that the festival plays within that thought experiment, and (more importantly), the role of the child. What is Le Guin trying to say with this story, particularly about moral reasoning? Finally, would the story have been better served had Le Guin returned to discuss the festival, or would such a decision have been for the worse when compared with the path she took?