Can anyone expand on why this technique isn’t more common? I’m so sick of seeing folks reinvent HTTP (poorly) on top of WebSockets. I get if extreme low latency is (allegedly) a requirement.
Hard answer: WS has been around for longer, and it's had more "marketing" for lack of a better word -- more people know about it and know how to use it. Retooling existing WS-using code to use HTTP/2 pipes would be a considerable effort with little to no perceived benefit to most users and teams.
Speculative answer: most web developers live closer to the application and presentation layers, and there's resistance to learning technologies that involve HTTP connection management e.g. in nginx (not to say that there are none of these, there are just fewer of us). WS was at the right place at the right time with a good high-level interface available to the client, and gained traction because of this.
https://bugzilla.mozilla.org/show_bug.cgi?id=906896 https://bugs.chromium.org/p/chromium/issues/detail?id=275955
Because WebSockets is 10 years old and HTTP/2 is 5 years old, and that's not including support in major frameworks for SSE.