I think that there might be a couple of reasons for this construction in Le Guin's short story. One reason might be that the individuals of Omelas are so immersed in happiness that they do not require an intermediary. There is such complete interaction with all aspects of their environment that this extends to the realm of religion and spirituality in the citizens of Omelas. A more sinister reason why there is no clergy might extend to the treatment of the child. The citizens of Omelas recognize that their happiness is contingent on the treatment of the child. The lack of the child's suffering means their own happiness is compromised. In this, religious clergy might be compelled to speak out against such a condition in their interpretation of spiritual identity and the presence of the divine. If there is no religious clergy, then the citizens of Omelas are able to completely understand their own reality as something that does not need or require excusing or explanation. The presence of the clergy might initiate a discussion that the people of Omelas are simply not capable or seeking to have. In this, the absence of the clergy allows for the Status Quo in Omelas to continue and prosper.