It’s hard to start a city from scratch. Most people don’t want to live somewhere that feels dead, and these new cities sometimes lack the jobs and commerce needed to support those who would live there. In Kangbashi, the government used some administrative tricks to address this, relocating bureaucratic buildings and schools, then trying to convince people in surrounding villages to move in. It had minor success. Today, a city designed for at least 500,000 has around 100,000 inhabitants.
I love that China actually tried to build massive cities. Without planning to populate them. Nice.