Here's why:
* Transitive verbs require a direct object. They describe an action that is done *to* something.
* Passive voice focuses on the *receiver* of the action, not the *doer*.
Example:
* Active voice: The dog chased the ball. (Transitive verb "chased" takes "ball" as the direct object)
* Passive voice: The ball was chased by the dog. (Focus shifts to the ball, the receiver of the action)
In essence, the passive voice rearranges the sentence structure to highlight the object that is acted upon.