The reason is one. And it is because ad blockers actually do not affect both the publishers and ad companies that much. About only less 5% of the internet users world wide have ever employed ad blockers (http://www.channelnewsasia.com/news/business/in-bringing-ad-...). I am sure they will move ad serving to the backend when they get better reasons to shun of client side technology. Its just not worth it....yet.