Every Saturday, thousands of runners worldwide participate in parkrun – a free five-kilometre run/walk event. Delivery relies on volunteers, and parkrunners are encouraged to volunteer regularly. However, volunteer recruitment is often difficult, and this study aimed to investigate the underpinning reasons for this. Data were collected from 6,749 parkrunners using an online survey, including 860 who had never volunteered. In addition to demographic information and views on incentives, non-volunteers were asked to rate agreement level for 18 statements about not volunteering. Main reasons were preferring to run or walk and not having got round to it. Principal component analysis indicated four underlying factors: inertia, self-interest, lack of knowledge and anxiety. Non-volunteers were younger, slower runners and walkers, less frequent parkrun attendees and had become involved more recently than volunteers. Ease of signing up to volunteer and being unsure of the commitment required were barriers. It is recommended that these two issues are addressed to help volunteer recruitment.