Leaks of server private keys probably do happen. But they don't get as much publicity.
When a password database has leaked, many sites decide to force users to change their password. Obviously this generates more publicity than if the site silently replace their secret key without forcing users to change their password. Depending on your point of view, this may make sense or not. There are certainly scenarios where a leaked private key would make it much more important for users to change their password than a leaked password database would.
There is a few reasons why leaking a password database is more likely to happen than leaking a private key.
First of all SSL may be terminated on separate hardware from the actual application. A weakness is more likely to be present in the application than in the SSL implementation due to the application often being more complicated than the SSL protocol and due to the application not having been reviewed as carefully.
If a weakness is found in the application, but SSL is terminated on another computer, then that weakness won't give access to the private key. Moreover even if it was on the same computer, then in some deployments the private key is placed in secured hardware such that even the SSL implementation doesn't have full access to the secret key.
Additionally password databases can leak due to SQL injection vulnerabilities, but those don't give access to the SSL secret keys.
If the secret key is leaked, the administrator can contact the CA to have the certificate revoked. In principle that will stop the threat posed by the leak.
However as soon as the secret key has been leaked, it could possibly be abused in mitm-attacks. Such a mitm-attack could give the attacker access to passwords in clear text. That makes password obtained this way easier to exploit than one from a leaked password database, because anybody with just a bit of a clue will hash passwords before storing them.
On the other hand using brute force on a leaked password database is likely to reveal at least a few clear text passwords, and those can be abused without needing to mount a mitm-attack in the first place.
So there are reasons for requiring users to change their password in either case, which of the two incidents would be worst depends on many other factors including the strength of the password. A strong password will remain secure even in case of a leaked password database (unless the password database is stored unencrypted), but if a mitm-attack is performed using a leaked private key, the strength of the password makes no difference.