That thread pre-dated browser enforcement of CT for DV certificates. My thinking then was that if browsers weren’t making use of it, I didn’t want to bulk up our certificates unnecessarily.
I suspect at the time I was also hoping that we could incentivize very widespread adoption of OCSP Stapling, which would improve privacy and reduce dependency on our OCSP resolver. I may have figured that if we made SCTs available over OCSP, that would be a strong encouragement, once browser enforcement did roll out, for subscribers to implement stapling.
Since then we’ve found that stapling is poorly implemented in a lot of web browsers, and concluded that we’re not likely to see most subscribers implementing stapling.
And also, of course, browsers started enforcing CT. When that change was announced, we wanted to make sure all of our subscribers’ certificates would keep working without onerous interventions, so we chose the X.509v3 extension approach, as did most CAs. We decided that we didn’t want to implement an option to leave the SCTs out of the cert and deliver them in OCSP, because this would have added considerable complexity to our issuance and OCSP signing pipelines for very marginal gains.