Why cruisers did not reinforce the Dunkirk rear guard

Why were cruisers not sent to support the Dunkirk rear guard with naval gunfire?

They could have helped, but the Royal Navy held them back because no one requested their deployment.

Cruiser gunfire could have provided useful support to the forces holding the Dunkirk perimeter. The Royal Navy nevertheless kept its cruisers back rather than assigning them to the evacuation beaches. The explanation is institutional: if the commanders ashore had requested the ships, the Navy would have sent them. No such request arrived, so the cruisers remained outside the operation. Available capability can go unused when coordination and demand signals fail.