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.
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.