I have read that it is a form of guilt and sour grapes. I order to not admit their privilege, people who grew up in suburbs say they are bad. And people who couldn't afford suburbs say they are bad. And rich people who can afford a comfortable life in a city, they are right but jerks.
I grew up in suburbia, and I remember fresh air, open spaces and low crime. I get that suburbs can be boring for some people, but a lot of people I knew growing up chose to return to the suburbs after living in bigger cities, or never left.