State of the art federated authentication is overly complicated by solutions bundling enterprise features such as resource authorization and directory services. These additional features should be factored out since they can be layered.
By contrast, BrowserId's parsimonious take on federated authentication (via an email handle) is refreshing.
I don't think it was because of a lack of belief in Persona and the value of pursuing federated identity. I would guess that it came down to priorities.