Why did mosaics become the most popular art in Byzantium in the 6th century?

Mosaic has become the most popular art in Byzantium since the 6th century. Mosaic is widely used to decorate the walls and floors of churches and patios and secular buildings. In mosaic painting, the purpose of color is not to imitate the natural color of the object, but to set the color according to the theme, that is, to have a fixed color scheme for some specific characters, so that the audience can know who is represented in the painting at a glance. For example, Christ uses blue or cherry red, sometimes with a golden base, and the Virgin Mary uses blue. Saints use yellow or blue, Sao Paulo uses purple ..., but in a picture with many people or backgrounds, pay attention to the color effect of the whole picture, and use complementary color juxtaposition or color block contrast. Indeed, Byzantine mosaics make use of all the rich changes of color spectrum to achieve brilliance and harmony, flashing incredible gem-like charm.