And because it's a layer 7 thing, so it only required updating the server and client software, not the OS... and only the client and server endpoints and not the routers in between... and because we only have two browser vendors who between them can push the ecosystem around, and maybe half a dozen relevant web server daemons.

Layer 3 of the Internet is the one that requires support in all software and on all routers in the network path, and those are run by millions of people in hundreds of countries with no central entity that can force them to do anything.

HTTP->HTTPS is only similar in terms of number of users, not in terms of the deployment itself. The network effects for IP are much stronger than for HTTP.

They don't "sorta" use v6, they're properly using it, and you can certainly go v6-only. I'm posting from a machine with no v4. Also, if you want to go there: HTTPS was released before IPv6, and yet still no browser is HTTPS only, despite how much easier it is to deploy it.