Because Governments can make laws but ICANN can’t? (GDPR explicitly exempts personal data that is legally required to be disclosed from protection)

Edit: actually after a bit more thought, it’s more nuanced than my somewhat glib response lets on (since there’s also an exemption for contractual obligations).

The answer is likely that a) the law hasn’t been tested in the courts under GDPR and/or b) the Government requirement has the additional test of providing a service, which may alleviate the privacy cost sufficiently.