Speakers, staff, and even people looking to increase their credibility in a particular field by networking and socializing all have very good reason to consider a convention to be a “workplace environment”. Even if they’re volunteering, even if they’re there only semi-professionally or as a hobbyist, the existence of a solid harassment policy that includes reporting mechanisms and collection of data for future improvement can be nothing but a good thing for their ability to carry out their work.
Conventions are not unique among workplaces — very many workplaces involve the dissemination of ideas, social components where “customers” can interact with one another and with the “employees” alike, and might even have an “overtime” component where people who are otherwise co-workers can fraternize outside of the purview of the actual “workplace environment”, often with those aforementioned “customers”. Most workplaces already have very solid harassment policies, and HR departments to enforce those policies. So why all the pushback against these policies when presented in context of trying to improve the situation for women who have apparently increasingly abandoning certain skeptical events despite leaders’ efforts to improve the situation?
My best guess is, because the people pushing back against these policies are the first ones who will be impacted by them.