It is said that the infrastructure required to sustain suburbs costs more money than these suburbs bring in. In the US a city gets federal money to build new suburbs, but when it comes to renewing them after a few decades, the city has to pay on its own and it often has not enough money to do it.
I am by no means an expert on this topic and just poorly summarized the contents of this video: https://youtube.com/watch?v=7IsMeKl-Sv0