I think it is because setting up a system like Social.coop requires a lot of effort, technical prowess, and patience. It's great for people who are deeply aligned with democratic practice. But for people more focused on other things, it's a high price to pay for founders to have less control in the end.
I think more widespread democratic practice online will require a) making starting up comparably easy to feudalism, and b) establishing norms that it is the right thing to do.