Board members of a nonprofit are subject to external supervision, including auditing, regulators and—in extreme cases—lawsuits. The Supreme Court is unique because it is the court of last resort for the entire United States—even a state supreme court justice would be able to have their recusal decisions reviewed by a higher court, much less a random board member of a nonprofit.

There are strict regulatory rules for 501c3 nonprofits (for instance - no private benefits for nonprofit insiders) and guidelines on how to implement them, for instance, via CoI policy, which we have: https://www.irs.gov/charities-non-profits/form-1023-purpose-...