Google makes money off search, which requires that users want to visit websites. All websites using HTTP are not secure. Unsecure websites are uninteresting to most users, but most users don't have the know-how to distinguish what sites are using HTTPS and which aren't. So the simplest solution is to get all websits to switch to HTTPS before it becomes a problem

Another possibility is Google is in an industry that makes money by collecting information about users, and by supporting universal HTTPS, they gained a competitive advantage over ISPs and others regarding user data for Google searches and other services.