People instantly perceive monoculture as a bad thing. One reason is fear, the fear of being hammered down if one sticks one's head up or strays too far from the proscribed trends. There's enough fear around to produce a lot of fads. While the idea that everyone should dress alike is obnoxious, it's inevitable that some people won't want to stick out, that children will demand to wear what the other kids wear, that dance crazes will sweep through night clubs around the world, and that everyone will be reading Chicken Soup for the Soul the same week. Governments seeking to increase their sphere of influence and corporations seeking the largest possible markets all pursue strategies that push uniformity of economy, uniformity of ideology, and uniformity of taste.
Fear is exploited by advertising and other outside pressures.
The impulse to maximize profit thrives on the demise of local traditions and cultures. If everyone has the same tastes, it's much easier to find a popular flavor to peddle. But other human impulses keep sameness from taking over. Social evolution, with its fashions, its cliques, its subgroups, favors complexity just as biological evolution does. The constant media barrage may push us to look like Kate Moss and drive sport utility vehicles, yet over time the forces that push us towards diversity have stubbornly held their own.