Hulu is attractive to advertisers because as far as streaming goes, it's a popular place to go to stream shows like The Office and 30 Rock. The problem, according to a report by MediaWeek is that Hulu doesn't actually get to sell that inventory.
In fact, according to sources, Hulu isn’t able to sell much inventory from NBC’s prime-time shows these days. That’s because—like a child grabbing for his or her favorite toy and shouting “Mine!”—NBC has recently begun pulling back the maximum allowable amount of ad inventory within its shows on Hulu, as are its sister cable networks like USA and Bravo.
Hulu’s other broadcast partners, ABC and Fox, are following suit, according to buyers familiar with Hulu’s operations, though NBC has been the most aggressive as they look to capitalize on heavy demand for video and assert more control over their assets. Hulu is a joint venture among NBC Universal, News Corp. and Disney.
According to the story, networks can hold back 100% of the inventory on two or three specified shows and as much as 85% of the inventory on other shows.