Over at FastMail we seriously looked into implementing Persona across the board. We're one of the bigger "small" email providers and we figured that it would be a good thing to get in on the ground floor if it succeeded, and have a(nother) feature to differentiate us from our competition, and to be able to give feedback on the system from the iDp perspective.
The hard requirement for HTTPS or DNSSEC is what raised the bar too high for us in the end (see https://github.com/mozilla/persona/issues/1523 for more info). Basically, the domain owner needs to securely delegate to the identity provider. Since we provide DNS and basic web hosting for most user domains, that means we have to provide HTTPS certificates for every domain we manage (and at least one IP per domain) or be able to serve proper DNSSEC records for every domain we manage (difficult when many registries we use still don't support it).
DNSSEC is something we're working towards, and I'd really like to have full support available this year. HTTPS without needing one IP per domain and multiple certificates is still not yet feasible, though there are specs gradually coming down the pipeline for it (DANE, DNA, POSH, etc). Without all this tech in place, Persona seems to be a non-starter for a IdP that wants to manage lots of domains.
I don't blame the Persona guys for this. I know they tried and they got a lot of it right, and should be applauded for that. Maybe the next round of federated authentication will work. I have no idea, but I know we'd still like to be involved, and we'll be watching the space with interest.