Here's why:
* Transitive verbs require a direct object. They show an action that is done *to* something or someone. Examples:
* "The dog chased the ball." (chased - transitive verb, ball - direct object)
* "She wrote a letter." (wrote - transitive verb, letter - direct object)
* Non-transitive verbs do not have a direct object. They show an action that is done *by* someone or something, but not done *to* anything. Examples:
* "The baby slept soundly." (slept - non-transitive verb)
* "The birds sang sweetly." (sang - non-transitive verb)
The passive voice is formed by using a form of "to be" and the past participle of a verb. It focuses on the action being received, rather than the person or thing performing it.
Since non-transitive verbs do not have a direct object to receive the action, there is no way to structure a sentence to put the action in the passive voice.
Example:
* Active voice: "The baby slept soundly."
* Passive voice is impossible because there is no direct object to "receive" the action of sleeping.
You can only use the passive voice with transitive verbs because they have a direct object that can become the subject of the passive sentence.