I would say that a big part of the issue is the difficulty in transferring one's account. Ignoring the fact that one simply can't transfer their posts, trying to manually copy all previously subscribed communities to a new account is a rather tedious task. I am aware that there exists scripts that can automate that process, but I don't think that it's fair to expect that the userbase should run 3rd party scripts. Until account transfer is properly implemented, defederation will continue to be a major issue.