In order to form a utopia, everything has to be perfect. Unfortunately, human beings aren’t perfect, and will never be. That goes especially when we’re left to their own devices. Humans are lazy, evil, greedy and any other multitude of negative synonyms going against a perfect, good populace.
People in power – usually governments or dictators – will force their vision of a perfect world upon the populace. Choice will be removed, because choices lead to mistakes that lead to erosion of the utopia. And this utopia is not necessarily a utopia – it’s the dictator’s vision of one.
This dynamic plays out across all (good) dystopian novels. In Uglies by Scott Westerfeld, people are turned into Pretties and made docile. In The Hunger Games by Suzanne Collins, the Capitol keeps people in check with a mix of entertainment and fear. In The Giver by Lois Lowry, citizens are made to suppress emotion by forgetting hardship and taking pills.
Utopias are inherently dystopian by nature not just because of the vision they seek to realize, but rather because of the means through which that vision is realized – a.k.a removal of choice and freedom.