From CyberWurx Customer Wiki

Using sv_password in l4dX causes your server to not show up in server browser requests as well as queries.

Please note, your server may not operate to its full potential with sv_password set. To work around this, use your steam group as a "private" entity. Setting your Steam Group ID and Turning off the Lobby System can help you create a private server for your steam group.

[edit] Breakdown of sv_password

sv_password is a server cvar used for password protecting servers. These servers will require a password to access. Because of the unique design of Left4Dead, this feature does not work properly and may cause issues while using your server.