Why River frigates displaced Castle corvettes

Why were River-class frigates favored over Castle-class corvettes?

Rivers were larger, tougher, and more standardized escorts, giving them more operational margin than the smaller Castle-class corvettes.

River-class frigates were larger and tougher than the visually distinctive Castle-class corvettes. Their frigate classification reflected a more substantial, more standardized escort. Greater size gave the design more room for endurance, equipment, and wartime growth. Appearance or charm mattered less than the operational margin available in a convoy escort.