"Why isn't HTTPS everywhere yet?": Because it makes absolutely no sense for the vast majority of online content. Email in https ? Sure. Reading news sites in https ? Accessing RFC in https ? My favorite online recipe site in https ? A total waste of money and CPU. Oh, and sure, the problem has been "solved" for few geeks using the latest browsers accepting let's encrypt certificates. Sure.
Hey, not so fast with conclusions, please.
Do you want an ISP filling your recipe reading with their advertisements. Nope? Or a random dude spoofing a Starbucks WiFi hotspot and serving you a malware on what you think is a trusted download site. Still no?
That's what TLS is for. Not for random paranoid geeks or "just email and online payments" security.
Oh, but, yes, Let's Encrypt is not yet completely production grade and still have some compatibility issues with older systems. Currently the only non-paid options I know are WoSign and StartSSL.