Why the Crown Colony class became a wartime Town-class derivative

Were the Crown Colony cruisers strictly a wartime design?

No. They were begun before the war as a reduced Town-class design intended for the conflict Britain expected was coming.

The Crown Colony class was a stripped-down development of the Town-class cruiser. Work began before the formal outbreak of war because British planners already believed conflict was approaching. More time might have allowed additional Towns before the reduced design became necessary. The class shows how a navy can shift from an ideal peacetime ship to a more economical wartime derivative before mobilization is publicly complete.