Because the faster response is negligible when you're an indie web host that can serve content over a single connection with reasonable speed. Where HTTP/3 really "shines" is when you connect to a web site that then has dozens of connections to other hosts (internal or external)...which is facebook/google/ad companies.

HTTP/3 speeds up that kind of content by reducing connection startup times to all of them, which can be compounded.

but it doesn't help for different external connections, each one needs a new connection.

it's good for multiple resources from the same host, maybe you have a lot of images on your photo blog or something

http3 has enhancements that speed up initial handshakes for new connections (QUIC essentially is UDP plus combining TLS and a native reliability layer to replace separate initial SYNs for them separately). So when you go to a page with tons of ads/trackers/javascript libraries, you're not left waiting as long for the ads to serve while browser reaches out to all those components and does separate TCP, TLS, and HTTP connections.

But users of most other websites won't see a ton of noticable benefits unless they have facebook/netflix/google levels of traffic. And at that point, you're either highly focused on the end user component code or you've outsourced it to a CDN that'll do the http3 for you anyways.