The animals rights group PETA is known for its shock tactics and pushing the envelope with its ads. Past ads that networks refused to air include the "veggie sex" ad slated to run during the Superbowl, and Pamela Anderson's stripping airport cop video. This ad, however, doesn't use sex or naked people to make people squirm.
According to PETA's blog:
When we first submitted our newest commercial to NBC in the hopes of running it during the Macy's Thanksgiving Day Parade, the station asked us to give more information about the cruelty behind turkey slaughter to back up the statements made in the ad. But even after we sent the network this New York Times article chronicling the grisly facts about turkey factory farming, it nixed the ad, claiming that "this commercial does not meet NBC Universal standards."