Why are newly molted crabs called soft-shell crabs?

Newly molted crabs are referred to as soft-shell crabs because they have recently shed their rigid exoskeleton through a process known as ecdysis, leaving them with a new, temporary covering that is soft and flexible.123

Because a crab's hard shell prevents growth, the animal must periodically undergo molting to increase in size. During this process, the crab secretes a new, soft exoskeleton beneath the old one. Once the old shell splits and the crab emerges, it expands its body by taking in water. The new cuticle remains soft and pliable for a period of time before it undergoes hardening through tanning and the deposition of [calcium carbonate]. During this vulnerable post-molt stage, the crab is characterized by its soft, fragile exterior.2314