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Re:Chrome 68 Released: HTTP Sites Marked as “Not Secure” by tmikaeld on Wednesday July 25, 2018 @09:39AM (#17609123)
"Let's Encrypt has been a godsend to me during this change."

I think that's what made Google push this, since they are on the board of advisors for the Let's encrypt certificate authority.

Re:Chrome 68 Released: HTTP Sites Marked as “Not Secure” by nsgi on Thursday July 26, 2018 @02:38PM (#17619763)
It's more likely that Google's support for HTTPS everywhere caused them to do both. It introduced HTTPS as a ranking signal in 2014, well before Let's Encrypt.