The answer is one click away in their FAQ:

Why do you charge for attendance? Several reasons:

• Organizing conferences costs time and money, like many other things in the Fediverse. We believe the sooner the Fediverse finds a way to pay for itself that doesn’t depend on the heroic efforts of a small number of individuals, the better. We want to build something that is sustainable, and to do that, it needs to know how to pay for itself.
• It aligns the objectives between organizers and attendees, and keeps us organizers focused on what you want out of the conference; as opposed to us pushing an agenda, or letting a sponsor undule influence the agenda.
• In our long experience organizing conferences we have learned that people who pay for an event are much more likely to actually attend. Free-of-charge conferences often have more no-show’s then actual attendees.