Because they fail to provide a convincing reason for non technical people/average Joes to switch over. Yes decentralisation is good, yes privacy is good, but on

Because they fail to provide a convincing reason for non technical people/average Joes to switch over. Yes decentralisation is good, yes privacy is good, but on their own they're not really selling points for a lot of people, and nor is 'allows more freedom of speech than $MAJOR_BRAND'. Most fail because they don't succeed on at least one of the following things: 1. A system selling feature that Facebook doesn't/can't offer 2. A decent UI/UX 3. An audience large enough to contain at least some of the user's friends/acquaintances