1. It matches what the CAA accounturi field has
2. Its consistent across an account, making it easier to set up new domains without needing to make any API calls
3. It doesn’t pin a users key, so they can rotate it without needing to update DNS records - which this method assumes is nontrivial, otherwise you’d use the classic DNS validation method
I didn't realize the email field wasn't persisted. I assumed it could be used in some type of account recovery scenario.