Why do porn sites have lower PageRank?

What are the technical reasons porn sites have such low PageRank? None go over PR6. A lack of links from trusted sites? Rampant link exchanges with low quality sites? Penalties for affiliate links? Or is there a general penalty on the industry? Studio 3X, New York Have a question? Ask it in our Webmaster Help Forum: www.google.com Want your question to be answered on a video like this? Follow us on Twitter and look for an announcement when we take new questions: twitter.com More videos: www.youtube.com Webmaster Central Blog: googlewebmastercentral.blogspot.com Webmaster Central: www.google.com