Biometrics are highly problematic in this context. IF someone steals your private key, you can jump through some hoops and get a new one issued, and regain control over your identity.
If someone steals your biometric data (and that is a thing that can happen through a variety of methods), there's no form you can fill out be issued new retinas. Your identity is permanently compromised.
A moderate increase in security[1] in the average case in exchange for catastrophic failure modes isn't good tradeoff.
(And given the very, very troubled history of biometric security, I'm being charitable assuming it's even an increase.)
