Light cruisers displaced protected cruisers from the battle fleet

Why did the Grand Fleet have armoured cruisers but not protected cruisers?

Because light cruisers had taken over the fleet duties formerly associated with protected cruisers, leaving the older ships to perform other work.

The label protected cruiser does not map neatly onto a permanent place in the battle fleet. By the First World War, newer light cruisers performed the scouting and fleet tasks that mattered in the Grand Fleet. Older protected cruisers could still exist while serving elsewhere or in secondary duties. This watch point helps readers follow functional succession across changing cruiser categories instead of treating each label as an isolated class.