Getting a contextual ad network going that can compete with Adsense is tough for two reasons:
1) Publishers are best served when there are many advertisers.
2) Advertisers are best served when there are many websites in the network.
This "network effect" maximizes the quality of match between ad and page, increases the probability of a click and ultimate conversion.
I saw this in reverse when I tested YPN: the same poorly matched ads kept appearing over and over, clicks were few, and eCPM was terrible.