Answer: Because Omelas is a utopian society
Step-by-step explanation:
A utopian society is an ideal world, which means guilt has to be erased so it won't turn into a dystopian society, which means some people benefit more than others, and guilt is one thing that can cause that.
A utopian society can never exist in real life, but dystopian societies are everywhere, some obviously way more than others.
Hope that helped a lot.
:)