Why the campaign uses light cruisers instead of destroyers

Did you make the Chester-class light cruisers function like destroyers?

Yes. Faced with a choice between light cruisers and destroyers, the campaign built light cruisers and gave them more torpedoes.

The campaign imposed a choice between building light cruisers and building destroyers. It favoured light cruisers with additional torpedoes so they could perform part of the destroyer attack role. The decision bought gun power and survivability in a larger hull, but it also helps explain the shortage of dedicated destroyers noted earlier in the stream. The substitution still leaves gaps in cost, numbers, screening, and specialized duties.