It's a fair question but an unanswerable question unless you can survey those who have left. It must be hard for those of us who see this a a great opportunity , to fathom those folks who abandon ship, or are no longer participating. Let's assume that like most of the population there are a variety of reasons which would lead to someone leaving. Life has a way of distracting and prioritizing the world differently for each individual. Some justifications are real and some are imagined.