Why do I need a separate IP address per domain that I want to secure?

You need to have a separate IP address for each domain you want to secure. The reason for this is because a certificate is bound only to a domain name but, the SSL protocol is bound to static IP addresses; therefore, any certificate-enabled web site must have its own unique IP address.The IP can be real (routable) or internal (RFC 1918 non-routable address) but, it must be unique on a server.
Courtesy of GeoTrust.com!