Depends. If the "why" was covered in the class material, then probably you're expected to memorize/regurgitate it. If not, if only the facts were covered, then it takes some thinking skills to select and distill the facts into a "why" narrative.
There also is no one "why", it's subjective. For example, what were "Hitler's biggest mistakes"? Ask 5 historians, and you'll get 5 different answers. A grade would be based on how well and insightfully you justified your answer.
There also is no one "why", it's subjective. For example, what were "Hitler's biggest mistakes"? Ask 5 historians, and you'll get 5 different answers. A grade would be based on how well and insightfully you justified your answer.