Emperor G built a beautiful walled city, inviting everyone in, encouraging them to paint their houses whatever color they like. A year later, Big G banned blue houses. If you didn't like the rules, you were more than welcome to build your house outside the city and paint it whatever color you like.
Only most people don't leave - blue is just one color and they weren't very interested in it anyway. Besides, the city is so beautiful and provides for their every need. In the coming years, people who want to paint their house blue badly enough to leave paradise are heavily scrutinized and eventually considered outcasts.
Over the years, more and more colors are slowly banned, one by one. People start to notice and complain once their favorite color is outlawed. But decades have passed since Emperor G's generous invitation. Entire generations have lived, died, and raised children inside the city. No one knows how to navigate the wilderness anymore. And even if they could, why would they want to? Thorns and weeds have overgrown the wasteland; it's much safer to stay inside the city walls. Besides, it's cozy and we have everything we need in here.