[yardstick]

Technical reason: Multiple ports = multiple apps.

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.