There are a considerable number of therapists in my area influenced by Buddhism and using mindfulness directly in therapy. I know many Buddhists who are either analysts or psychiatrists.

But you are right that it isn't so common. I remember going to a panel on trauma and asked why they were all so secular, what about spirituality, and no one had an answer. Perhaps there is a fear that first of all it should be a science and having religion in there would make it less so? Also I think that people don't want to "push" their own beliefs onto others in therapy, so that old idea of neutrality comes back in.