Because all secrets are assigned codes and, I believe, names and registered through a central secrets database coordinated through all services. Someone has to close the ticket, first, and there may be outstanding issues.
I think you mean code-word clearance. This is for strictly limited-access programs, where the number of people knowing a secret is controlled. Normally, you would have a security clearance plus need-to-know. These programs put the weight much more heavily on the need-to-know side.
i.e. You may have a top-secret clearance and know that the Derkderkistani military is buying new tanks, but you don't need to know that the source of that info was from an agent in their Ministry of Finance.
