It's documented well already. The Overton window for pornography has continued to move to more and more extreme forms as what was once considered unacceptable and taboo becomes socially acceptable. It's because there is a market for deviance. Some people are interested in what's taboo and off limits and so long as they are approaching or just crossing that line, they're happy. As we've moved that line these people are no longer happy with the status quo and want content that is taboo, so a new market forms around that.

Pornographers know this and talk about. Read David Foster Wallace's essay on it.