Lengthy Wall Street Journal report on the travails of Apple's iAds mobile advertising program. Unfortunately, they don't offer a lot of explanation until the very end of the story:
One major challenge Apple faces: because the company only sells ads that appear on Apple devices, marketers are forced to buy ads from competitors to reach broader audiences, says IDC analyst Karsten Weide. "Apple we believe will, over time, fade into the background," he says. "It was attempted to make sure that even consumers advertising experience on Apple devices was perfect, but it hasn't really worked."
-WSJ.com, December 12, 2011
It's another example where Apple's refusal to make its services multi-platform has hurt the company. I'd cite iBooks as another leading example.