My guess is that it’s indeed mainly the stigma of illegal drugs and not wanting to lose credibility as an organisation by promoting alternative treatments from that category that haven’t been approved. Probably not status quo bias as there’s enthusiasm about new official treatments. Some cluster headache patients themselves may be reluctant to try illegal drugs, but desperation and the encouragement of other patients in the community reduce the psychological barriers.
So, results of the small-scale migraine study I mentioned above were actually published in June and showed a significant effect of psilocybin on migraines. “Preliminary Analysis of the Sustained Effects of a Single Low Oral Dose of Psilocybin in Migraine Headache”, https://headachejournal.onlinelibrary.wiley.com/doi/10.1111/head.13854 The results were also shown at a cluster headache conference I spoke at over the weekend: a single low dose of psilocybin cut migraine frequency in half. Pretty encouraging!