Random ideas off the top of my head, but I’m sure there are more:

• they believe in the platform and know they have the skills to help it grow
• they’re tech enthusiasts and it’s basically hobby time
• general curiosity about how it all works from the admin side, some people love to learn new stuff
• spotted a gap in the market for an instance around a particular theme
• they want a solo / friends and family only instance to avoid defederation issues and generally curate the feed
• they want control over their own data and experience
• they happen to own the most perfect domain name and just need something to do with it