Volunteers aren’t free labor. They’re paid in feeling useful. Run out of that and they stop coming.

People who are not being paid are still getting something, and what they are getting is the sense that their afternoon mattered. Hand them an hour of standing around, or a job anybody could have done, or no idea what the point was, and the wage disappears. Nobody quits over money they were never promised. They quit over feeling pointless.

Give whoever turns up a job that visibly matters, and tell them what it made possible. Say thank you with a specific detail rather than in general. Then ask what they would rather be doing next time.