This depends on the child. Some children are extremly creative and want to express themselves through clothing. Other feel insecure about themselves and want uniforms to make themselves look smart and improve their confidence. Other kids simply like the way some uniforms look. To really know you would have to ask the child.
usually some girls prefer to dress down, because they are girls (no duh), while others don’t like picking out what to wear so they can just wear the same thing over and over. usually guys like to wear uniforms because they don’t like picking out their clothes either.