Here's why:
* Cunning is an adjective describing a person or animal that is clever, sly, and skillful in deception.
* Cunningness is the noun form of cunning, representing the quality or state of being cunning.
While "cunning" can sometimes be used as a noun, it generally refers to a specific instance of clever trickery. "Cunningness" is more general and refers to the abstract concept of being clever and deceptive.