StoutFiles - 12:53 pm on Sep 4, 2012 (gmt 0)
It is almostly completely due to oversaturation. Back in '03, there were few players and plenty of wealth to spread around. Now everyone and their mother has a website with AdSense and advertisers can't keep up.
The other major factor is that Google went public in '04, and every year since they have likely been siphoning off money from AdSense to keep their shareholders happy. It's pretty easy to do when they won't share any relevant numbers with you; everything about how you're getting a paid remains a mystery.