With the, as they point out in above letter, "hyper-infectious Omicron variant", you simply cannot host a safe audio show. It is much more infectious than previous variants, and once one person has it, it's everywhere in the crowd. The organizers cannot and do not want to take the risk.
The virus may be milder than previous variants, and most vaccinated people will not fall seriously ill, but for those with underlying diseases and those who are immunocompromised it may be a different story. I will not comment on the unvaccinated, in order to avoid politics.
it may be 'venue' enforced. either the municipality or Hotel. i have no personal knowledge, just a guess. or maybe the tipping point was reached with exhibitor cancellations.
something not entirely in their control.
and the unknown spector of Omicron is a different viewpoint than last spring/summer prior even to the Delta surge.
i'm not in any way agreeing, only that there is much not known.