This is a basic represention and inclusion issue. Unless you are actively seeking out voices of those minorities and addressing their concerns you will have a reinforcing loop where behaviour that puts people off engaging will continue and it will continue to limit people from those minorities being involved (and in the worst case causing active harm to some people who end getting involved). From what I understand the behaviour that has been demonstrated and from who those people leaving it is clear this is active issue within Nix. Having a diverse range of people and perspectives will actually make the outputs (software) and community generally better. It's about recognising the problems in the formal and informal structures you are creating and working to address them.
Additionally, but just to clarify nepotism would be giving positions based on relationships with people in power and not ensuring that your board contains a more representative set of backgrounds and perspectives.