I don't know if there is any technical reason which prohibits this, but it would make it easier to scan down the list of questions and communicates more information than [closed], which could have been applied for a number of different reasons.
Examples from the current top questions:
- Icon idea for “project” [closed: offtopic]
- Silverlight + Expression Blend = Good UX? [closed: not constructive]
- Bringing computer icons into the future: Is it too disruptive to UX? [closed: exact duplicate]