Why older cruisers were candidates for aircraft conversion

Why would C- and Danae-class cruisers be converted rather than E-class cruisers?

The E class remained useful in secondary cruiser work, while the older C and Danae classes were easier to repurpose for anti-aircraft or aircraft-related roles.

E-class cruisers retained more utility in secondary roles than the proposed conversion candidates. The older C- and Danae-class designs had less remaining value in ordinary light-cruiser work. They could instead be repurposed for anti-aircraft or aircraft-support duties. Conversion therefore becomes an opportunity-cost decision: alter the hulls whose existing combat role is easiest to surrender.