Because Facebook managed to be the first massive service that convinced just about everyone to sign up. A good chunk of those people would much rather deal wit

Because Facebook managed to be the first massive service that convinced just about everyone to sign up. A good chunk of those people would much rather deal with the minor annoyances or nebulous complaints about Facebook than set up and learn how to use a new service. Just being "a bit better in some ways" is not enough to drive the mass adoption needed to create a shift away from the incumbent Facebook. And since Facebook would never allow some theoretical decentralized network to interface with their service, it would end up like all the other attempts: people who like to try new things/dislike the status quo will check it out, realize they still need to cross-post to Facebook, and give up. The only way to pull it off would be to offer something sufficiently novel and in-demand that people would scramble to use it. From what I've seen in the past, those sorts of things don't come from loose federations of decentralized network nodes. They come from commercial development and heavy promotion....or they are bought up by a company that does those things.