I would say its becuase it was a utopian place. Everything needed to be perfect in order to stay utopian. Being different can ruin the "perfect" look. This is because people can hurt others based on what they look like. people who look the same won't necessarily bicker with each other
For example...
the book mentions that twins aren't a thing in their world. The idea of two people who look alike would confuse others therfore creating a stir in peoples minds. This is why they get rid of one of the two twins.
Another example is color..
they get rid of people's ability to see color so that there isn't a way for anyone to be different other that basic looks and age
One possible theme could be “how a person is affected by growing older”. Shakespeare compares old age to the seasonal shift. The narrator also describes how aging is harsh and ultimately comes to see its inevitability. Hope this helps!
