"soft-shell crab is a prized dish in many ocean-side resorts". why are the crabs' shells soft?

Soft-shell is a term that is used in the culinary field. Crabs shell softens when their exoskeleton and remain soft from the shedding. Crabbers with the knowledge of their field know when crabs are prime to catch with a soft-shell in order to keep the shells from hardening.

what is this I might know it:)