Leak monitoring would be a service provided by the UNS, not falling to a volunteer, and credential revocation could be automatic and immediate.
I suppose we sort of have that bolted on with OpenID/OAuth, but that's still 'choose a provider' rather than 'this is the one way', with many servers run by different entities, but one 'system'.
1) most people don't want their information public and searchable to that extent
2) most orgs _want_ to silo you in or otherwise control your account
3) the org using x500 still needs to have their own permissions separate from the central directory, which is the harder part of auth[nz], so just rolling your own authn is often easier.
I think you're absolutely right in particular with #2.
But if it had come originally, alongside DNS, 'everything has an address, everyone has an identity', it might've been an unquestionable fact of the internet.
Orgs can't silo you in to their alternate net where they have a more desirable domain name, because it's just not practical or user friendly.
I just think it might have been so for user identity.