I’m oversimplifying here, but it’s because it requires conformity. Everyone has to change themselves to fit the system, and if they can’t or don’t, then they are generally not treated very well.
The polar opposite problem is when there’s too much individual freedom and people are allowed to harm others with impunity, so once again some people will be treated very poorly.
Reminds me of an all evil campaign I played in once.
I recall “heroically” leading the towns militia into a wererat den on the night of the full moon, only to lock them all in together and finish off the stragglers on both sides afterwards. Turns out militiamen’s scalps look awfully similar to those of reverted wererats, and we turned quite a profit.