In a word, because it's social. B2B marketing norms are based on professional / commercial interactions - not social interactions. B2B marketers (I count myself here) have operated on a different basis - our interactions, while they can become personal, are a little more formal and a little less frequent than those in the social realm.
So when you tell a B2B marketer to use social media it's a little bit like telling them to take their clothes off.
But the evidence that social media works even in B2B is there. Adoption will be slow, but it will happen.