One possibility is that HTTP/2 is much more work to implement, and since it isn’t /absolutely/ required for base “serve a thing” functionality, it maybe be left to be implemented later.
I'm not sure that "HTTP/3 is about to overtake HTTP/2" is a very fair statement given how many years it took load balancers to start supporting HTTP/2.
HTTP/3 (QUIC) is genuinely exciting and I'm very eager to see it. But given it's even more different from http/2 than http/2 is from http 1.1, it's probably gonna be another 3-4 years before we see good load open source balancer performance in front of it.
If that was true, I'd be convinced by it. But is it true? HTTP 1.1 is a pretty big and complicated spec.