The ones I saw... I'd say many of them were lured by the promise of tightly controlled relationships conducted in private. There are no witnesses. If it goes badly, the client can be eliminated and quickly replaced. Down the memory hole. And the relationships are structured such that the therapist is built up and becomes the center of attention. This is probably irresistible for some who feel powerless or inferior. It's all so creepy.

eta: By "some who feel powerless or inferior" I mean therapists.