Because they are often started by a small group (usually just one) of people who are passionate about the subject enough to work on it in their spare time, so t

Because they are often started by a small group (usually just one) of people who are passionate about the subject enough to work on it in their spare time, so they have stronger personal involvement. That provides the drive, but also the side-effect of personal drama when things go wrong. Larger institutions usually setup protections against just that kind of drama, but often with the side-effect of reducing motivation.