Eh, I believe that to some extent. Someone decided that Black Sabbath would be more popular than The Pixies. But I don't think anyone decided that shared style

Eh, I believe that to some extent. Someone decided that Black Sabbath would be more popular than The Pixies. But I don't think anyone decided that shared style of music would become popular: that was a function of sentiment at the time. From the perspective of a hypothetical overlord, controlling what people want costs money. It's cheaper to predict what people want and sell them that. Of course if you predict wrong, you have to exert some control to make sure you recoup your investment. But music isn't an industry where you have to manufacture demand, so it's cheaper not to.