The reason is very technical, and a somewhat "chicken and egg" problem. The SSL protocol layer stays below the HTTP protocol layer and encapsulates HTTP. When an SSL connection (HTTPS) is established Apache/mod ssl has to negotiate the SSL protocol PARAMETERS with the client. For this, mod ssl has to consult the configuration of the virtual server (for instance it has to look for the cipher suite, the server certificate, etc.). But in order to GO to the correct virtual server Apache has to know the Host HTTP header field. To do this, the HTTP request header has to be READ. This cannot be done before the SSL handshake is finished, but the information is needed in order to complete the SSL handshake phase. SEE the next question for how to circumvent this issue.

Note that if you have a wildcard SSL certificate, or a certificate that has multiple hostnames on it using subjectAltName fields, you can use SSL on name-based virtual hosts WITHOUT further workarounds.

The reason is very technical, and a somewhat "chicken and egg" problem. The SSL protocol layer stays below the HTTP protocol layer and encapsulates HTTP. When an SSL connection (HTTPS) is established Apache/mod ssl has to negotiate the SSL protocol parameters with the client. For this, mod ssl has to consult the configuration of the virtual server (for instance it has to look for the cipher suite, the server certificate, etc.). But in order to go to the correct virtual server Apache has to know the Host HTTP header field. To do this, the HTTP request header has to be read. This cannot be done before the SSL handshake is finished, but the information is needed in order to complete the SSL handshake phase. See the next question for how to circumvent this issue.

Note that if you have a wildcard SSL certificate, or a certificate that has multiple hostnames on it using subjectAltName fields, you can use SSL on name-based virtual hosts without further workarounds.