There's probably a technical reason I'm unaware of, but why are you allowed to have HTTP and HTTPS handled differently (besides then encryption portion)?
HTTP and HTTPS use different ports (TCP 80 and 443). You can run one web server application on the HTTP port serving content A, and a completely different web server application on the HTTPS port serving content B. With firewalls doing NAT port translation this could even result in HTTP requests going to a completely different machine than the HTTPS requests.
From a non-tech reason, there are some types of sites that the HTTPS content should never be available on a HTTP site. For example an online payment form. In such cases a sensible website will either disable HTTP entirely (and use a subdomain for secure content, rather than the top leve/ www. domain), or have a basic HTTP site that transparently redirects to the HTTPS version.
Servers can really do whatever they want, based on whatever they want. They can serve a different page to clients based on e.g. "Accept-Language": "en" might be a completely different page than "fr", rather than just a translation.
That's just how it happened. And historically, it was common to put only the "important things" behind TLS. That never really made sense from a security perspective, but it certainly saved CPU cycles.
