When a CA has an incident, such as learning that one of their domain validation methods is flawed, they need to be able revoke all certificates impacted by the incident. Without a reliable database of certificates, there's no guarantee they'll find all of the impacted certificates. OCSP could fail closed in this situation. CRLs always fail open.
Aside: it was in fact envisioned that OCSP could be used to detect CA compromises, and the BRs used to say "The CA SHOULD monitor the OCSP responder for requests for 'unused' serial numbers as part of its security response procedures." I'm not sure how many CAs actually implemented that, and in any case I don't think it ever detected any compromises.
Aside: it was in fact envisioned that OCSP could be used to detect CA compromises, and the BRs used to say "The CA SHOULD monitor the OCSP responder for requests for 'unused' serial numbers as part of its security response procedures." I'm not sure how many CAs actually implemented that, and in any case I don't think it ever detected any compromises.