Why can't I BCC users on tickets in Zendesk?
In Zendesk BCC's are not recognized and are not supported in tickets. When a user is added as a BCC on an inbound email, the user will not be added or acknowledged anywhere in the ticket or events. Additionally, BCC's are not notified of any ticket updates from the agent interface.
When the BCC'd user responds to the original email it may be captured through the system's inbound email threading processes causing the message to be appended to the existing ticket in the form of a private note.
For more information, see the article: Using CCs, followers, and @mentions.