[verandaguy]

>Can anyone expand on why this technique isn’t more common?

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.