I ran a training for therapists on alternate relationship structures, most had never heard of polyamory, most felt it meant something had gone wrong for people to be in that situation.
Personally, I think all the things which can make polyamorus relationships difficult also exist in monogamous relationships.
Generalisation or fear on my part perhaps, but I imagine a large percentage are those who are coming from a religious background. I've often wished I could filter them out when searching for therapists.
When I was looking for a therapist, that was a huge struggle for me too. Even some of the providers under the LGBTQ+ filters were proudly religious, a few proudly displaying their degrees from religious institutions.
Reasons I developed a "therapy is an expensive waste of my time" mindset after living in FL for a decade. I'm not paying $200 a visit for some pompous shitbag to tell me I'm sad because I don't simp for Jesus