It's probably a complicated web of business relationships and strategies that is the answer for why Google would be reluctant to make such a change. It would cause a lot of upheaval and potentially ill will from companies affected by the change.
It would be an extremely bold move by Google to make to prevent search leakage by default. Does Google believe that encrypted.google.com appeases the users who are concerned? Does Google believe that it is only a minority of users that are concerned?