It's simple issues.
In first answer, Google Search BOT::
Google engine will search HTTP first, if there's HTTP and HTTPS exists in the sametime.
Google will store same contents then, HTTP data will be stored.
It means less computing and network Bandwidth consumes.
they search and caching tremendous sites and datas. so, 1% of more cost means huge counts of costs.
Second, StackExchange service providers' view::
Most of stack exchange contents is view on public. most of writer acts on this sites. but, that's not aim to exclusive view. they wants to share informations to public. no login, will read this-sites contents.
I think this kind of Service property will stay in HTTP and HTTPS sametime.
Just Login-time , user account and pw entering time, SSL needs.
more of considerations::
If your websites and what websites for advertisement, no needs personal secure access, then why use SSL (HTTPS).
they want to publish their sites to more public, and spread their information anonymously, and that case , they thinks make their websites more easy to access and more easy to read what they want to release.
But, most of websites has another authentication mechanism, when log-in , they switching background to support https and make secure sessions.
I think that's depend on websites contents.
Another aspect of view,
When using SSL, the websites needs to spend more cost, because it needs more server capabilities and more server resources. it makes more cost per connections.
So, they makes websites not to use SSL.
In case of 1~x concurrent connection thru SSL is ignorable. but, there's over xK conccruent connection , then more server costs.
And then, most of IDC or service provider adapt SSL Offload or That Kinds of Expensive devices.