Because all they've published so far is API specs and fluffy PR sites that try to portray it as "oh so much better" without offering any insight about why it is better. They can claim "more privacy" all day long, but without any details about what gets stored where and why it is supposed to be safer, they don't make a compelling case.
Look at this page for example: https://login.persona.org/about (the "how it works" page) - it has 0 details about these claims and unfortunately, we're already tired of reading how Google and FB respect our privacy. From "outside", it looks like we need to give Mozilla our (existing) credentials and trust them to handle them with care. Why should we? I feel safer making pwgen passwords for every new site I need to register at.