Why has pop culture been such a pervasive force in society?
Pop culture is a pervasive force in society simply because it is what people want. Since people want it and like it, they consume it. Because they consume it, it ends up influencing them.
Popular culture consists of the sorts of ideas and attitudes that are held by the mainstream of a society and are, therefore, represented in popular entertainment such as music, movies, and television. This can be contrasted with "high culture" like Shakespeare and operas and fine art.
Popular culture is pervasive because it reflects what the average person wants. It is not what people are taught in school and not what they are "supposed to" like. It is what they actually do like. Because of this, people watch TV and movies much more than Shakespeare plays. They listen to pop music rather than opera. This gives pop culture an opportunity to be a force in society in ways that are out of the reach of high culture.