I'm curious on why the upgrade mecanism to tell the server you understand and prefere HTTP3 is different from the one used to upgrade to a Websocket connection?

I'm curious on why the upgrade mecanism to tell the server you understand and prefere HTTP3 is different from the one used to upgrade to a Websocket connection? It doesn't seem to use the "Connection: Upgrade" and "Upgrade: XXX" headers but rather "Alt-Svc"... I'm a total newbie on the topic, so I'm curious about this.