> Unfortunately, we don’t have the freedom to expand other standard protocols the same way.

There's nothing standing in your way. However, SSH has the advantage that it's commonly used interactively, so FIDO is a good fit. Any protocol that is most often used silently with the user maybe not attentive or not even present won't work well - it's easy for your phone's mail client to just present the same password it remembers each time it re-connects, but it would be annoying if you need to touch the fingerprint reader each time. I can imagine (if anybody wanted to) retro-fitting SMTP submission to do FIDO, although I don't know if there's a practical way to hack it into the existing SASL AUTH layer.