The main motivation here is the desire to have a seamless transition to newer protocols without having to change the site. This way, your hosting service can en

The main motivation here is the desire to have a seamless transition to newer protocols without having to change the site. This way, your hosting service can enable HTTP/3 and it will just work with an HTTP/3 compatible browser. This is the way every upgrade in TLS and HTTP has worked for quite some time. It's especially important with HTTP/3 because there are network environments where HTTP/3 doesn't work and so you need to fall back to HTTP/2, and you obviously want that to work seamlessly. In addition, the basic security structure in the Web is the origin, defined in RFC 6454, and origins are defined by a triplet of [scheme, host, port]. It would not be good to have HTTP over TLS and HTTP/3 be different origins. For instance, you don't want to lose all your cookies if the site turns HTTP/3 on or off.