https is overrated. No ranking boost whatsoever, everyone who says there's a "subtle" boost is probably mistaking normal positive fluctuation with it. It does fuck all tbh... also, letsencrypt is bound to get a ton of spammers thanks to it being free and not having to jack shit to get an SSL. It won't be long until these are completely worthless. The green lock is nice, but its a nice to have... not necessary.
Google should just stick to what they do best and focus on ranking good websites instead of everyone's security. Like some mentioned, probably something to do with large advertisers not being able to serve over encrypted pages.