A big part of this appears to be that google massively downranks any non-https sites-- most old sites, which are just documents and have no accounts or writing functionality have had no reason to adopt https.
If you run an older internet reference, please add https. The search engines also have an obvious 'recent' bias, but everything we can do to prevent the loss of accessibility of older reference material helps.
If you run an older internet reference, please add https. The search engines also have an obvious 'recent' bias, but everything we can do to prevent the loss of accessibility of older reference material helps.