The ultimate reason is "no one has felt strongly enough about having this feature up to this point in time to contribute it." By and large fossil is used by individual projects. Just because someone is interested in SQLite doesn't mean they are interested in fossil or pikchr, or any other combination. What you are looking for is a first class citizen of github or chiselapp, and great for people who want to offload their project to a third party. For those who don't want to do that, there isn't such a feature yet. Whether or not your questions motivate someone to create "social project functionality" above and beyond what fossil already has (which has apparently satisfied most people up to now) remains to be seen.